1. Introduction: Inflectional Distinctions

Most natural languages make inflectional distinctions: distinct forms of a lexeme reflecting the different roles that it may play in syntax and in its contribution to the semantic composition of the sentences in which it appears. Consider, for example, the forms of the French verbal lexeme conduire ‘drive’ in (1). The forms conduisons and conduisez are used in the present indicative, one with a first-person plural subject, the other with a second-person plural subject; the forms conduisons and conduirons are both used with a first-person plural subject, one in the present indicative, the other in the future indicative.

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As these examples suggest, the distinctions among a lexeme’s inflected forms amount to differences in morphological form that correlate with differences in morphosyntactic content (differences in tense, person/number agreement, and so on). Thus, the inflectional distinctions among the forms of a lexeme L may be represented in a table in which each cell (a) corresponds to a unique combination of morphosyntactic properties with which L may be associated in syntax and (b) houses the form of L that realizes that combination of properties. A complete table of this sort is a representation of L’s inflectional paradigm; for example, conduire has the inflectional paradigm in Table 1. Canonically, sameness or difference of content among a lexeme’s inflected forms is invariably expressed as sameness or difference in their morphology (cf. Brown, Chumakina, & Corbett, 2013; Corbett, 2005, 2009); that is, a canonical paradigm has exactly one form per cell and a different form in each cell. Nevertheless, deviations from this canonical pattern are extremely common; for instance, a defective paradigm has one or more unrealized cells, an overabundant paradigm has alternative forms in one or more cells, and a syncretic paradigm has the same form in more than one cell.

Table 1. The synthetic paradigm of French conduire ‘drive’

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The twin tasks of defining a language’s inflected forms and of identifying the grammatical dimensions of the inflectional paradigms that house them raise of a number of fundamental questions about the nature of morphology and its interfaces, which I address in turn in the sections that follow:

2. Distinguishing Inflection From Derivation

A long-standing concern in morphology is the need to differentiate between inflectional distinctions among forms of the same lexeme and derivational distinctions among forms realizing separate but related lexemes (Anderson, 1985; Booij, 2000; Stump, 1998, 2005). The forms conduisons and conduisez are both forms of the verbal lexeme conduire, but the form conducteur is customarily associated with a separate, nominal lexeme conducteur ‘driver’ deriving from conduire. Practical criteria such as those in (2) have been invoked to differentiate between inflectional distinctions and derivational distinctions.

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These criteria do, however, vary in their reliability. As it is stated, (2a) is a sufficient but not a necessary criterion; derivation may fail to produce a change in syntactic category (read ⇒ reread) or in lexical meaning (syntactic ⇒ syntactical). What’s more, (2a) is at odds with the usual assumption that a verb’s participles and gerunds are among its inflected forms, even though participles frequently have adjectival characteristics and gerunds, nominal characteristics.

Criterion (2b) is substantially more reliable, though it, too, raises questions. For instance, the distinction between different tenses is generally held to be an inflectional distinction, but in many languages, different tenses aren’t obviously tied to different syntactic contexts. For instance, the context John ___ cookies admits either bakes or baked. One might attempt to argue that the sequence-of-tense phenomenon is a syntactic phenomenon that is sensitive to tense differences, causing the tense of a higher clause to spread to a subordinate clause (as when John is saying, “I bake cookies for a living” is reported as John said he baked cookies for a living). But the sequence-of-tense phenomenon is less obviously a syntactic phenomenon than a semantic or pragmatic one: John said he bakes cookies is also possible, and its lack of tense sequencing affords it subtly different pragmatics.

Criterion (2c) is at most a statistical generalization. Defective paradigms are, precisely, inflectional paradigms with gaps, and highly productive derivation may approach gaplessness.

Criterion (2d) often separates inflection from derivation along the same lines as other criteria, but it doesn’t always. For example, there are some derivational operations that are extremely regular in their semantics; an example is the derivation of ordinal numerals in -th. At the same time, the phenomena of deponency (§7.4) and metaconjugation (§7.5) involve inflected forms with unexpected semantics; somewhat less spectacularly, there are occasional instances in which an inflected form is lexicalized with a special meaning, as in the case of brethren.

Criterion (2e) entails that in a word’s morphology, any inflection should constitute an outer layer. On first consideration, this seems like an accurate generalization: In a complex word like norm-al-iz-ation-s, the outermost suffix is inflectional, the remaining suffixes are derivational, and the word as a whole is no longer open to additional morphological operations. The notion that inflection follows derivation has therefore occasionally been accorded the status of a theoretical principle, namely, the so-called Split Morphology Hypothesis (Perlmutter, 1988); ultimately, though, this hypothesis proves to be empirically untenable (Bochner, 1984, Booij, 1994, 1996; Stump, 1990a, 1990b). In fact, marks of inflection may appear internally to derivation in certain circumstances.

First, category-preserving derivation gives rise to headed expressions (Stump, 1995, 2001, p. 96ff), and the head of such a derivative may be the locus of inflectional marking. For example, Southern Barasano [Tucanoan, Colombia] has a category-preserving suffix -aka for deriving diminutive nouns (Smith, 1973). This may apply both to ordinary count nouns (cotɨ ‘pot’ ⇒ cotɨaka ‘little pot’) or to mass or collective nouns (hoa ‘hairs’ ⇒ hoaka ‘little hairs,’ oho ‘bananas’ ⇒ ohoaka ‘little bananas’); when one of these diminutives is inflected for number (either through pluralization or singularization), the number inflection is internal to the diminutive suffix, on the head noun (cotɨ-ri-aka ‘little pots,’ hoa-bą-ka ‘little hair,’ oho-ro-aka ‘little banana’).

There are also instances in which category-changing derivational operations apply to inflected forms; English has sporadic examples of this sort (worsen, betterment), but this can be a robust phenomenon as well, as in Breton, where the denominal adjective suffix -ek applies to plurals when it is semantically appropriate (e.g., drein-ek ‘thorny’ from drein, plural of draen ‘thorn’; kerniell-ek ‘having horns’ from kerniel, plural of korn ‘horn’; preñved-ek ‘wormy’ from preñved, plural of preñv ‘worm’).

The less than full reliability of the criteria in (2) has led some morphologists to doubt the theoretical validity of the distinction between inflection and derivation (e.g., Bochner, 1992, p. 12ff; Di Sciullo & Williams, 1987, p. 69ff). A more prevalent view is that inflection and derivation are indeed distinct, but that this distinction only emerges clearly in the context of a carefully articulated complex of assumptions about the architecture of grammar, the lexicon, and their interface (Aronoff, 1994; Beard, 1995; Spencer, 2013; Stump, 2001, 2016).

3. Kinds of Inflectional Exponence

Exponence is the relation between an inflectional marking (an exponent) and the content that it expresses. An exponent may be concatenative (e.g., the -ed in walk-ed) or nonconcatenative (e.g., the ablaut in sing → sang). Logically, a variety of different kinds of exponence relations are imaginable (Coates, 2000; Matthews, 1972). The simplest of these is biunique exponence, in which (a) each exponent expresses a single morphosyntactic property and always expresses that same property, and (b) each morphosyntactic property is expressed by a single exponent, and always by that same exponent.

But relations between exponents and their content may also be one-to-many: In instances of cumulative exponence, properties in different inflectional categories are systematically expressed by a single exponent, for example, an exponent that expresses case and number together. Exponence relations may also be many-to-one: In instances of extended (or multiple) exponence, a word form has more than one exponent expressing the same content, for example, a verb form in which past tense is expressed both by ablaut and by suffixation. In instances of inflectional allomorphy, the same content is expressed by different exponents in different word forms, for example, by ablaut in one word form but by suffixation in another; and in instances of inflectional polyfunctionality, the same exponent expresses different but related content in different word forms, for example, a first-person singular affix that codes a possessor on nouns but expresses subject agreement on verbs.

An inflected word form’s inflectional exponence consists not only of its inflectional markings, but of their ordering. Ordering relations among affixes are most directly observable, but even nonconcatenative operations follow a particular order of application in their definition of an inflected word form’s realization. The nature of such ordering relations (particularly as they apply to affixes) is an object of vigorous debate. Can the relative ordering of affixes be deduced from their semantic scope (Baker, 1985; Bybee, 1985; Rice, 2000)? Or are ordering relations essentially arbitrary, describable only in templatic terms (Good, 2016; Stump, 1997)? Why are the ordering relations among inflectional affixes rigid in most languages, but flexible in others (Bickel et al., 2007; Luutonen, 1997; Noyer, 1994)? What is an appropriate model for the representation of ordering relations (Crysmann & Bonami, 2016)? A comprehensive account of these issues remains to be proposed.

4. Periphrasis and Clisis

Traditionally, inflectional exponents have been assumed to involve purely synthetic morphology; thus, Table 1 lists the synthetic forms of conduire. But the morphosyntax of French verbs also involves periphrasis; for example, a verb’s ‘passé composé’ is formed by combining an auxiliary (avoir ‘have’ or être ‘be’) in the present indicative with the verb’s past participle: avons conduit ‘(we) drove,’ sommes allés ‘(we) went.’ Is avons conduit one of the inflected forms of conduire?

Recent research suggests that periphrases do in fact constitute inflected forms; for discussion, see Ackerman and Stump (2004), Bonami (2015), Börjars et al. (1997), Chumakina and Corbett (2013), Sadler and Spencer (2001), and Spencer (2003). One kind of evidence for this conclusion is the complementary relationship that exists between synthetic morphology and periphrasis in the morphosyntax of some category of lexemes. In some instances, the same content that is expressed synthetically in the inflection of one subclass of lexemes is expressed periphrastically in the inflection of another subclass. In Sanskrit, for example, it is usual for a verb to inflect synthetically in the perfect tense; but certain verbs instead exhibit a periphrastic inflection in which the verb, marked with a special suffix ‑ām, joins with perfect forms of the verb as ‘be’ (Stump, 2013). Table 2 illustrates, with the synthetic perfect active inflection of budh ‘awaken’ alongside the periphrastic perfect active inflection of bodhaya ‘cause to awaken.’ If budh and bodhaya are both assumed to have perfect paradigms, then one must assume that the paradigm of bodhaya includes cells realized by periphrasis rather than by synthetic morphology.

Table 2. Perfect active paradigms of two Sanskrit verbs

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Although the complementarity of synthesis and periphrasis has a lexical dimension in this case, there are other instances in which this complementarity has a purely morphosyntactic basis. Latin verbs, for example, inflect both for perfectiveness (perfective or nonperfective) and for voice (active or passive): Active forms are invariably synthetic, as are nonperfective forms, but perfective passives are periphrastic; the paradigm of parāre ‘prepare’ in Table 3 illustrates. As this example suggests, all Latin verbs that have perfective passive cells in their paradigm realize these cells periphrastically.

Table 3. Indicative forms of Latin parāre ‘prepare’

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A related issue concerns the status of clitics in inflectional morphology. In French, verbs serve as hosts for pronominal and adverbial clitics: Jean vous y conduira ‘Jean will drive you there.’ Are host-clitic groups such as vous y conduira inflected forms of conduire? Recent proposals for treating at least some clitics as inflectional morphology include Bonami and Boyé (2007), Bonami and Samvelian (2008), and Miller and Sag (1997). As in the case of periphrasis, one often finds a kind of complementarity in the use of synthetic morphology and that of clitics. In the expression of prenominal possessives, personal pronouns inflect, usually suppletively (I / my, you / your, we / our), but other nominals employ a phrase-final clitic (Dale’s, someone else’s, a guy I know’s); here again, there is a lexical dimension to the alternation of synthetic inflection with clisis.

But the complementarity between synthetic morphology and clisis may also be purely morphosyntactic in its conditioning. Consider, for example, the paradigms of the Shughni verbs wirīvdōw ‘stand’ (intransitive) and wīftōw ‘knit’ (transitive) in Table 4. In the present tense, these verbs inflect alike, invariably exhibiting a person-number subject-agreement suffix. In the past tense, the verbs inflect differently. The intransitive verb has two past-tense stems, a default stem and a masculine singular stem; the transitive verb employs a single past-tense stem that is insensitive to differences of gender or number. Person-number agreement is not expressed suffixally, but by a second-position clitic situated on the first constituent of the clause, whatever it might be; in these examples, it happens to be the subject pronoun. The intransitive verb, unlike the transitive, lacks a second-position subject-agreement clitic in the third singular of the past.

Table 4. Nonpast and past inflection of two Shughni verbs

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It is clear that Shughni verbs agree with their subject in person and number; what is interesting is that this agreement is expressed by synthetic morphology in the present but by a clitic in the past. Thus, it is reasonable to think of the cells of past-tense paradigms as having realizations that are most often discontinuous.

The notions of periphrastic inflection and discontinous realization present nontrivial problems for understanding the interface of inflection with syntax; for discussion, see Bonami and Webelhuth (2012), and Bonami (2015).

5. Modeling Inflectional Exponence

The simplest linguistic signs involve an arbitrary union of form and meaning. For example, the union of the phonological form /dɔg/ with the meaning ‘canine’ is purely arbitrary, and must therefore be stored in an English speaker’s memory. But words that are more complex are in general not fully arbitrary; for example, the union of /dɔgz/ with ‘canines’ is motivated to the extent that it parallels other form-meaning unions (e.g., that of /dɔg/ with ‘canine’ and that of /bɝdz/ with ‘avians’). A perennial debate among morphologists concerns the most insightful way to model such parallelisms in the form and content of inflected words. What, in particular, is the most insightful way of modeling inflectional exponence—the relation between a word form’s morphosyntactic properties and the morphological markings by which these properties are expressed?

Structuralist grammatical theory developed the idea that a word w can be exhaustively segmented into morphemes—nonoverlapping units such that (i) each unit has a form and a content, (ii) no unit can be further subdivided into units having both form and content, and (iii) the units’ form and content jointly constitute the form and content of w. The hypothesis that a complex word can be modeled as the sum of its component morphemes has been remarkably persistent.

Theoretical executions of this hypothesis have two important characteristics. First, they are lexical in the sense that at the simplest level, associations between form and content are stipulated in the lexicon as lists of morphemes; these basic associations then form the basis for the associations of form and content arising through the combination of morphemes. Second, morpheme-based theories of morphology are incremental: The form and content of a complex word are built up cumulatively and in parallel, through the combination of its component morphemes. In view of these characteristics, four kinds of phenomena constitute prima facie counterevidence to morpheme-based theories of morphology; I shall label these Problems 1 through 4.

Problem 1. The morpheme hypothesis’ focus on segmentability suggests that morphology is fundamentally concatenative; thus, morpheme-based theories treat concatenative morphology as primary and nonconcatenative morphology as a secondary effect triggered by a word form’s concatenative inflection. For example, a morpheme-based analysis of English verb inflection might assume that the -d suffix is the sole exponent of past tense in told, treating the vowel substitution e → o not as an exponent per se, but as the effect of an operation of ablaut or stem substitution triggered by the combination of tell with -d.

The assumption that nonconcatenative morphology is a secondary effect triggered by morphemic exponents does present a problem for morpheme-based theories of inflection, since some nonconcatenative operations are not overtly accompanied by a triggering morpheme; for example, if the vowel substitution in told is triggered by -d, it is not apparent what triggers the vowel substitution in broke. The gambit of postulating a zero past-tense morpheme to trigger the vowel substitution in broke may solve the problem from a technical point of view, but it inevitably raises issues of learnability. Why must a language learner postulate an inaudible exponent of tense to trigger the vowel substitution in broke rather than simply treat the vowel substitution itself as an audible exponent of tense?

More generally, the notion that morphology is fundamentally concatenative is at obvious odds with the widespread incidence of nonconcatenative marks of inflectional content; for example, concatenation of a noun’s stem with a plural morpheme is only one of the diverse ways in which languages mark plural number (Table 5).

Table 5. Exponents of plural number in nine languages

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Problem 2. The morpheme hypothesis suggests that no morpheme in a word’s structure should duplicate the content of one or more other morphemes in the same word, since that duplication would be without motivation, failing to effect any increment in the word’s content; yet, extended (or multiple) exponence—the incidence of two or more morphemes expressing the same part of a word form’s content—is extremely common. The inflection of nouns in the Noon language [Niger-Congo; Senegal] provides a clear example (Table 6). In Noon, nouns fall into six classes, and a noun’s membership in a particular class determines the agreement morphology of its modifiers. The nouns themselves inflect for number (singular or plural) and definiteness (indefinite or definite), and definite forms additionally inflect for location (near the speaker, near the hearer, or near neither interlocutor). Each location suffix is preceded by a class suffix signaling the noun’s class membership; in addition, nouns belonging to classes 4–6 carry the appropriate class prefix. Accordingly, forms such as p-ëlkít-p-ii ‘the thread near me’ exhibit extended exponence, with the p- prefix and the -p suffix both expressing singular number and class 5 membership. (See Harris, 2016, for extensive discussion of the phenomenon of extended exponence, with abundant evidence from a range of languages).

Table 6. Indefinite and definite forms of nouns in the six Noon noun classes

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Problem 3. The morpheme hypothesis entails that no part of a word’s content should fail to be associated with any of its individual morphemes. Yet, one commonly encounters instances of underdetermination, in which some part of a word’s content cannot be directly attributed to any one of its component morphemes. Consider, for example, the finite paradigm of the Bulgarian verb krad ‘steal’ in Table 7. In this paradigm, krádox is unambiguously the first-person singular aorist form, yet none of its component morphemes expresses either first person or singular number; note, in particular, that the suffixes -o and -x both appear in all three persons of the aorist plural.

Table 7. Finite forms of Bulgarian krad ‘steal’

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Problem 4. The morpheme hypothesis entails that inflected word forms have an internal hierarchical structure. The incremental nature of morpheme-based approaches to inflection relies on this structure to account for apparent scope relations among a word’s affixes; in passive verb forms in Sanskrit, for example, the suffixal mark of passive voice is internal to (has “narrower scope” than) the subject-agreement marker, whose agreement with a verb’s logical object presupposes a passivized stem: dṛś‑ya-nte [see-pres.pass-3pl] ‘they are seen.’ The underlying assumption here is that inflected word forms are compositional in the same way that phrases are—that just as the meaning of a phrase is a function of the meanings of its immediate constituents, so the meaning of each complex constituent in an inflected word form’s hierarchical structure is a function of the meanings of its immediate components (Baker, 1985). Counterevidence to this assumption is, however, widely observable; in Latin, for example, the verb form vide-nt-ur [see-3pl-pass] ‘they are seen’ exhibits “scope relations” opposite to those of Sanskrit dṛśyante, whose meaning it nevertheless parallels.

In view of these four apparent problems with the morpheme hypothesis, many morphologists favor theories of inflection affording an account of inflectional content and form that is neither lexical nor incremental. In lexical theories, the lexicon is regarded as an inventory of morphemes, from which each morpheme is individually inserted into an inflected word’s morphological structure. By contrast, inferential theories hold that the lexicon lists lexemes and their stems, and that inflectional markings are not inserted directly from the lexicon, but signal the application of rules that induce a lexeme L’s inflected word forms either from less complex stems or from other word forms realizing L. Because the relations defined by these rules may or may not be concatenative, they are compatible with the full variety of inflectional exponents, avoiding Problem 1.

In incremental theories, a word’s content is built up cumulatively from the content of the individual morphemes of which it is composed. By contrast, realizational theories hold that an inflected word’s content logically precedes its form, serving in effect to drive the sequence of rule applications that determine this form. In a realizational theory of inflection, it is perfectly possible for the same part of a word’s morphosyntactic content to trigger the application of more than one rule (in that way inducing extended exponence), and it is equally possible that part of a word’s morphosyntactic content may fail to trigger the application of any rule (so that that bit of content will be underdetermined by the morphology of the word’s form); in this way, realizational theories avoid Problems 2 and 3. In general, realizational theories in no way entail that an inflected word’s morphological structure should be isomorphic to its semantic representation; indeed, such theories are compatible with the view (Anderson, 1992) that an inflected word form’s internal structure is nothing more than its phonological/prosodic structure (and thus avoid Problem 4). Inferential-realizational theories therefore constitute a well-motivated alternative to lexical-incremental theories of inflection based on the morpheme hypothesis.

Current models of inflectional morphology differ with respect to the lexical/inferential and incremental/realizational dimensions (Stump, 2001). The family of lexeme-based models falling under the “word and paradigm” label are in general both inferential and realizational. Among these, some models induce a lexeme’s complex word forms “vertically,” from less complex stems (Anderson, 1992; Matthews, 1972,); another induces a lexeme L’s complex word forms “horizontally,” from other word forms realizing L (Blevins, 2006); and still other exploit both vertical and horizontal realization (Stump, 2001, 2016; Zwicky, 1985,). Models such as those described by Selkirk (1982) and Lieber (1992) are lexical and incremental. Steele (1995) employs a model that is inferential but incremental. Distributed Morphology (Halle & Marantz, 1993; Noyer, 1992) is lexical and in some ways realizational, though with incremental undertones. It is realizational in the sense that the insertion of an inflectional exponent (or ‘vocabulary item’) into a word form’s constituent structure is not what contributes the content that it expresses; that content is instead already present in abstract form in the structure into which insertion takes place, having been integrated into that structure by syntactic principles. At the same time, the assembly of a word’s morphosyntactic content by syntactic principles itself has an incremental flavor to it. In word and paradigm theories, no such syntactic assembly is necessary, since a word form’s morphosyntactic content is intrinsic to the definition of the paradigm cell that it realizes; but typically of incremental theories, Distributed Morphology attributes no theoretical necessity to inflection paradigms.

6. The Canonical Relation of Content to Form in Inflectional Paradigms

Identifying a lexeme L’s inflectional paradigm is an intricate matter, presupposing the identification of

Corbett (2009) describes canonical inflectional paradigms as conforming to the systematic set of criteria in Table 8. Column (a) specifies the canonical characteristics of an individual paradigm, and Column (b), the canonical characteristics of a system of paradigms belonging to the same syntactic category. By the first criterion, word forms within a paradigm are canonically alike in their morphotactics (e.g., they all have the form Stem-suffix), as are word forms in corresponding cells of distinct paradigms. By the second criterion, word forms within a paradigm canonically share the same stem, while corresponding word forms in distinct paradigms canonically have different stems. By the third criterion, word forms within the same paradigm canonically differ in their inflectional marking (e.g., walk-s, walk-ing, walk-ed), while corresponding word forms in distinct paradigms have the same inflectional marking (e.g., walk-s, sleep-s, move-s). The net effect of these criteria is that canonically, word forms belonging to the same paradigm are different, as are corresponding word forms belonging to distinct paradigms.

Table 8. Canonical inflection paraphrased from Corbett (2009, p. 2)

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If all inflectional paradigms conformed to these canonical properties, paradigms would be rather uninteresting things—indeed, their theoretical significance could be reasonably called into question. But inflectional paradigms are interesting and important precisely because they deviate from these properties so often; indeed, one would be hard pressed to find a single language whose paradigms didn’t deviate in various ways. These deviations, far from being mere descriptive annoyances, provide important evidence for the place of inflectional morphology in a language’s grammar.

7. Deviations From Canonical Paradigm Structure

7.1 Inflection Classes

Pattern (3) is a theorem of the canonical properties in Table 8. Many languages, however, deviate markedly from this pattern. For example, among the Latin nouns in Table 9, no two inflect identically in all 12 case-number combinations. These nouns are therefore traditionally assigned to the four distinct declension classes indicated in the table. A declension class (like a verb’s conjugation class) is an inflection class (Stump, 2015). If two lexemes inflect for exactly the same morphosyntactic property sets, but employ different morphology to realize these property sets, then the lexemes belong to distinct inflection classes. Thus, an inflection class is customarily seen as a class of lexemes that are alike in their inflectional morphology but that differ in their inflectional morphology from members of other such classes. Because inflection-class distinctions are only important for a language’s morphology (being in effect invisible for all other grammatical components), their incidence is one sort of motivation for the view that, notwithstanding its interfaces with other components, a language’s morphology is autonomous in its function.

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Table 9. Examples of the five traditional Latin declensions

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Although it is customary to think of inflection classes as classes of lexemes, they are more plausibly seen as classes of stems. The inflection of a lexeme may involve more than one stem, and these may or may not belong to the same inflection class. Consider the examples in Table 10. In the paradigm of dātṛ ‘giver,’ the stems dātār-, dātar- and dātr- (with its alternants dātur-, dātṛ- and dātṝ-) alternate according to the pattern of a single inflection class; but the paradigm of kroṣṭṛ ‘jackal’ is heteroclite: Some of its stems inflect as members of the class to which dātṛ’s stems belong, while others inflect as members of the distinct class whose members include the stems of paśu ‘domestic animal.’ Thus, kroṣṭṛ is a single lexeme, but its stems belong to more than one inflection class.

Table 10. The declension of three Sanskrit nouns

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The heteroclisis of kroṣṭṛ is a lexically isolated, irregular phenomenon (Stump, 2006). But there are also inflectional systems in which it is usual for a lexeme to have different stems in different inflection classes (Stump, 2016). Sanskrit verbs, for example, generally inflect in four different tense systems: present, aorist, perfect, and future. Each of the four systems has its own peculiar set of conjugation-class distinctions: according to the traditional classification, the present system has ten conjugations, the aorist-system seven, and the perfect and future systems each two; see Table 11. Although there are occasional correlations, it is in general not possible to deduce the class of a lexeme’s stem in one tense system from the class of its stem in another tense system. Thus, it is essentially a matter of lexical stipulation that the verb bhū ‘become’ has a present-system stem in the first conjugation (bháva-), an aorist-system stem in the root aorist conjugation (ábhū-), a synthetic perfect-system stem (babhū́v-), and a future-system stem in the -iṣyá conjugation (bhaviṣyá-); that the verb stu ‘praise’ has a present-system stem in the second conjugation (stā́u-), an aorist-system stem in the iṣ-aorist conjugation (ástāviṣ-), a synthetic perfect-system stem (tuṣṭā́v-), and a future-system stem in the -syá conjugation (stoṣyá-); and so on.

Table 11. Conjugation classes in the four Sanskrit tense systems

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Inflection classes introduce an important dimension of complexity into a language’s inflectional system. They engender what Ackerman and Malouf (2013) call the Paradigm Cell Filling Problem—language learners’ task of deducing a full paradigm of inflected forms for a lexeme only some of whose inflected forms they have actually encountered. In the simplest cases, a lexeme’s full paradigm is directly deducible from the phonology of its stem; in Moru [Nilo-Saharan; Sudan], for example, a verb’s conjugation-class membership is deducible from whether its stem is monosyllabic or disyllabic and (if it is disyllabic) whether it is vowel-initial or consonant-initial. But inflection-class membership may have grammatical correlates instead of or in addition to phonological correlates; in Sanskrit, for example, the phonology of the noun stem mātṛ- ‘mother’ limits its declension-class membership to a small number of choices, but the choice among these depends on its feminine gender. Very often, however, a lexeme’s inflection-class membership seems purely arbitrary. In English, for example, sit, slit, and knit belong to three distinct conjugations: The three-way past-tense distinction sat, slit and knitted is without rhyme or reason.

It has been widely hypothesized that there are limits on the difficulty of deducing a lexeme’s full paradigm from a subset of its forms. According to the No-Blur Principle (Cameron-Faulkner & Carstairs-McCarthy, 2000) and the Single Surface Base Hypothesis (Albright, 2002, 2008), a paradigm’s inflection-class membership is generally deducible from a single diagnostic form; there is, however, considerable prima facie counterevidence to this claim. Even so, a variety of empirical measures consistently reveal important limits on the difficulty of filling in paradigm cells (Ackerman et al., 2009; Milin, Kuperman, Kostić, & Harald Baayen, 2009; Moscoso del Prado Martín, Kostić & Harald Baayen, 2004; Stump & Finkel, 2013).

7.2 Morphomic Properties

Inflection classes are an example of what Aronoff (1994) labels morphomes: categories that have no grammatical or semantic significance beyond the confines of a language’s morphology. Inflection classes are morphomes that have a lexical dimension: intuitively, they classify lexemes according to the morphology by which their stems inflect for a shared system of morphosyntactic property sets, contrary to the canonical property (3). But morphomes may have a purely grammatical orientation, causing the realization of certain paradigm cells to conform to a common pattern despite their morphosyntactic heterogeneousness. Morphomes of this latter sort constitute a deviation from the pattern in (4), also a theorem of the canonical properties in Table 8.

(4)

Consider an example of this sort of deviation from Bena-bena [Gorokan; Papua New Guinea], whose verbs exhibit several suffix position classes. In the suffix position labeled ‘d’ in Table 12, the default suffix is -be; but in the second-person singular and the first-person plural, -ne instead appears. Thus, the content expressed by -ne is not morphosyntactically coherent, but is instead morphosyntactically disjunctive. One might suppose that this is a matter of pure historical accident, but other affixes exhibit this same disjunctive quality. For example, there is an emphatic suffix that appears as -na by default (as in bila-na yabe ‘they will definitely go’) but as -ta in the second-person singular or first-person plural (bila-ta yabe ‘you (sg.) will definitely go’); similarly, there is an interrogative suffix appearing as -fi by default (as in bilu-fi-he ‘shall I go?’) but at -pi in the second-person singular or first-person plural (bilu-pi-he ‘shall we (pl.) go?’). (See Young, 1964, for additional discussion). It is clear that the -ne, -ta and -pi suffixes express a morphomic property shared by second-person singular and first-person plural forms—a property that has neither morphosyntactic coherence nor syntactic or semantic significance but which is important for the language’s inflectional realization.

Table 12. Indicative paradigm of the Class A verb ho- ‘hit’ in Bena-bena (in compact and exploded forms; a, b, c, and d label affix positions)

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7.3 Defectiveness

In the canonical case, lexemes belonging to the same syntactic category have isomorphic paradigms, each possessing the same number of cells, distinguished by the same system of morphosyntactic property sets. There are, however, cases in which certain lexemes have a smaller than usual number of cells (Baerman, Corbett, & Brown, 2010; Karlsson, 2000; Sims, 2015). In some such instances, the deficit of cells has a reasonable semantic explanation; scissors, for example, has no singular precisely because it refers to a joined pair of blades. Yet, there are also lexemes whose cell deficit has no independent explanation—whose paradigms are simply defective. A striking example is the paradigm of the French verb traire ‘milk’ in Table 13.

Table 13. The paradigm of French traire ‘milk’

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7.4 Deponency

Canonically, the same morphology recurs with the same function; in the inflection of English verbs, for example, the suffix -s always expresses agreement with a third-person subject in the present indicative. There are, however, cases in which the same morphology has one function in one set of paradigms but a contrasting function in a different set of paradigms (Baerman et al., 2007). In Old Norse, for example, the inflectional morphology exhibited in the past tense by the strong verb fara ‘go’ is exactly that exhibited in the present tense by the verb munu ‘will,’ whose own past tense matches that of the weak verb dœma ‘judge’; Table 14 illustrates. The verb munu ‘will’ is one of a small number of so-called preterite-present verbs, whose present-tense inflections elsewhere serve as past-tense inflections. These preterite-present verbs are deponent (< Latin dēpōnere ‘lay aside’) in the sense that their present-tense morphology has ‘laid aside’ its usual past-tense meaning.

Table 14. Indicative forms of three Old Norse verbs (Shaded forms and boxed forms are alike in their inflectional morphology.)

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7.5 Metaconjugation

A phenomenon related to deponency is what Stump (2016) calls metaconjugation. This is the use of a particular inflection class to express different morphosyntactic properties in different paradigms. In the inflection of Sanskrit verbs, for example, both the present system and the aorist-system have preterite forms marked with the prefix a- (the so-called augment); these preterite forms express the imperfect tense in the present system and the aorist tense itself in the aorist-system. Strikingly, the thematic aorist-system conjugation employs the same morphology in the aorist as the sixth present-system conjugation employs in the imperfect. Thus, consider the verbs tud ‘strike’ and tuṣ ‘be happy.’ The verb tud has a present-system stem in the sixth conjugation (augmented form atuda-) and an aorist-system stem in the s-aorist conjugation (active form atauts-); the verb tuṣ has a present-system stem in the fourth conjugation (augmented form atuṣya-) and an aorist-system stem in the thematic conjugation (atuṣa-). As the forms in Table 15 show, the sixth present-system conjugation employs exactly the same preterite morphology as the thematic aorist conjugation. That is, there is a single conjugation that serves to express the tenses of the present system in the inflection of some verbs, but instead serves to express the aorist tense in the inflection of other verbs; this conjugation is labeled the sixth conjugation in the inflection of the first group of verbs and the thematic aorist conjugation in that of the second group.

Table 15. The imperfect and aorist indicative active forms of two Sanskrit verbs

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8. The Interfaces of Inflection

Among the most vigorous debates among contemporary morphologists is whether a language’s inflectional system is simply a part of its syntax or is instead an autonomous grammatical component whose interface with syntax is subject to tight restrictions. According to the assumptions of Distributed Morphology, words have a hierarchical structure comparable to that of phrases, and the shape of a word’s hierarchical structure is determined by independently motivated principles for defining and reorganizing syntactic constituents. A word’s structure may therefore comprise more than one terminal node in syntax; vocabulary insertion fills these terminal nodes with roots and affixes, so that inflected words are literally formed in the syntax.

Lexeme-based approaches to morphology involve a markedly different conception of the relation of inflectional morphology to syntax. In these approaches, a language’s morphology is an autonomous system that defines a whole word form for insertion into a single terminal node in syntax, subject only to the restriction that the morphosyntactic property set that conditions the word form's inflectional realization be nondistinct from the property set that the syntax associates with the node into which it is inserted. Word forms themselves have no internal hierarchical structure other than their phonological/prosodic structure, and their syntax conforms to the Lexical Integrity Principle (Bresnan & Mchombo, 1995).

A challenge for both of these conceptions of inflectional morphology is the wide range of ways in which a language’s inflection may deviate from the canonical characteristics postulated in Table 8. In Distributed Morphology, these deviations make it necessary to assume that a word form’s syntactic structure undergoes various kinds of preprocessing in advance of vocabulary insertion; these preprocessing operations such morphological merger, fusion, fission, impoverishment, copying, and others (Embick & Noyer, 2001; Halle, 1997; Marantz, 1988; Noyer, 1992).

In lexeme-based approaches to morphology, deviations from the canonical inflectional patterns in Table 8 are seen as mismatches between the contentive aspects of a lexeme’s paradigm and the inflectional realization of that content. Some such mismatches reflect a kind of poverty in a language’s rules of inflectional realization—specifically, a failure to give morphological expression to all of the content that is available for realization. Other mismatches reflect a kind of parasitic dependency among paradigm cells, such that the realization of one cell simply follows that of a contrasting cell. Still others involve morphology that realizes morphomic properties, or the exceptional realization of some property by the morphology that ordinarily serves to realize a contrasting property.

Stump (2016) advances the novel proposal that paradigms themselves are of two kinds: In a lexeme L’s content paradigm, cells are distinguished by property sets that are accessible to L’s syntax and semantics, while in a stem’s form paradigm, cells are distinguished by property sets that are available for morphological realization. On this view, the cells in a lexeme L’s content paradigm acquire their inflectional realization by virtue of their association with particular cells in the form paradigm of L’s stem. Accordingly, a language’s inflectional morphology must specify the mapping between the cells of a content paradigm and the associated cells in the corresponding form paradigm; canonically, this mapping is trivial (with each form cell sharing the morphosyntactic properties of the content cell that is associated with it), but each sort of deviation from canonical inflection entails its own peculiar complication of this mapping.