Conjugation classes have been defined as the set of all forms of a verb that spell out all possible morphosyntactic categories of person, number, tense, aspect, mood, and/or other additional categories that the language expresses in verbs. Theme vowels instantiate conjugation classes as purely morphological markers; that is, they determine the verb’s morphophonological surface shape but not its syntactic or semantic properties. They typically split the vocabulary items of the category verb into groups that spellout morphosyntactic and morphosemantic feature specifications with the same inflectional affixes. The bond between verbs and their conjugational marking is idiosyncratic, and cannot be established on semantic, syntactic, or phonological grounds, although there have been serious attempts at finding a systematic correlation. The existence of theme vowels and arbitrary conjugation classes has been taken by lexicalist theories as empirical evidence to argue against syntactic approaches to word formation and are used as one of the main arguments for the autonomy of morphology. They further raise questions on the nature of basic morphological notions such as stems or paradigms and serve as a good empirical ground for theories of allomorphy and syncretism, or to test psycholinguistic and neurolinguistic theories of productivity, full decomposition, and storage. Conjugations and their instantiation via theme vowels may also be a challenge for theories of first language acquisition and the learning of morphological categories devoid of any semantic meaning or syntactic alignment that extend to second language acquisition as well. Thus, analyzing their nature, their representation, and their place in grammar is crucial as the approach to these units can have profound effects on linguistic theory and the architecture of grammar.
Phonotactics is the study of restrictions on possible sound sequences in a language. In any language, some phonotactic constraints can be stated without reference to morphology, but many of the more nuanced phonotactic generalizations do make use of morphosyntactic and lexical information. At the most basic level, many languages mark edges of words in some phonological way. Different phonotactic constraints hold of sounds that belong to the same morpheme as opposed to sounds that are separated by a morpheme boundary. Different phonotactic constraints may apply to morphemes of different types (such as roots versus affixes). There are also correlations between phonotactic shapes and following certain morphosyntactic and phonological rules, which may correlate to syntactic category, declension class, or etymological origins. Approaches to the interaction between phonotactics and morphology address two questions: (1) how to account for rules that are sensitive to morpheme boundaries and structure and (2) determining the status of phonotactic constraints associated with only some morphemes. Theories differ as to how much morphological information phonology is allowed to access. In some theories of phonology, any reference to the specific identities or subclasses of morphemes would exclude a rule from the domain of phonology proper. These rules are either part of the morphology or are not given the status of a rule at all. Other theories allow the phonological grammar to refer to detailed morphological and lexical information. Depending on the theory, phonotactic differences between morphemes may receive direct explanations or be seen as the residue of historical change and not something that constitutes grammatical knowledge in the speaker’s mind.