Morphological change refers to change(s) in the structure of words. Since morphology is interrelated with phonology, syntax, and semantics, changes affecting the structure and properties of words should be seen as changes at the respective interfaces of grammar. On a more abstract level, this point relates to linguistic theory. Looking at the history of morphological theory, mainly from a generative perspective, it becomes evident that despite a number of papers that have contributed to a better understanding of the role of morphology in grammar, both from a synchronic and diachronic point of view, it is still seen as a “Cinderella subject” today. So there is still a need for further research in this area. Generally, the field of diachronic morphology has been dealing with the identification of the main types of change, their mechanisms as well as the causes of morphological change, the latter of which are traditionally categorized as internal and external change. Some authors take a more general view and state the locus of change can be seen in the transmission of grammar from one generation to the next (abductive change). Concerning the main types of change, we can say that many of them occur at the interfaces with morphology: changes on the phonology–morphology interface like i-mutation, changes on the syntax–morphology interface like the rise of inflectional morphology, and changes on the semantics–morphology like the rise of derivational suffixes. Examples from the history of English (which in this article are sometimes complemented with examples from German and the Romance languages) illustrate that sometimes changes indeed cross component boundaries, at least once (the history of the linking-s in German has even become a prosodic phenomenon). Apart from these interface phenomena, it is common lore to assume morphology-internal changes, analogy being the most prominent example. A phenomenon regularly discussed in the context of morphological change is grammaticalization. Some authors have posed the question of whether such special types of change really exist or whether they are, after all, general processes of change that should be modeled in a general theory of linguistic change. Apart from this pressing question, further aspects that need to be addressed in the future are the modularity of grammar and the place of morphology.
Analogy is traditionally regarded as one of the three main factors responsible for language change, along with sound change and borrowing. Whereas sound change is understood to be phonetically motivated and blind to structural patterns and semantic and functional relationships, analogy is licensed precisely by those patterns and relationships. In the Neogrammarian tradition, analogical change is regarded, at least largely, as a by-product of the normal operation (acquisition, representation, and use) of the mental grammar. Historical linguists commonly use proportional equations of the form A : B = C : X to represent analogical innovations, where A, B, and C are (sets of) word forms known to the innovator, who solves for X by discerning a formal relationship between A and B and then deductively arriving at a form that is related to C in the same way that B is related to A. Along with the core type of analogical change captured by proportional equations, most historical linguists include a number of other phenomena under the analogy umbrella. Some of these, such as paradigm leveling—the reduction or elimination of stem alternations in paradigms—are arguably largely proportional, but others such as contamination and folk etymology seem to have less to do with the normal operation of the mental grammar and instead involve some kind of interference among the mental representations of phonetically or semantically similar forms. The Neogrammarian approach to analogical change has been criticized and challenged on a variety of grounds, and a number of important scholars use the term “analogy” in a rather different sense, to refer to the role that phonological and/or semantic similarity play in the influence that forms exert on each other.
Indo-Aryan languages have the longest documented historical record, with the earliest attested texts going back to 1900 bce. Old Indo-Aryan (Vedic, Sanskrit) had an inflectional case-marking system where nominatives functioned as subjects. Objects could be realized via several different case markers (depending on semantic and structural factors), but not the nominative. This inflectional system was lost over the course of several centuries during Middle Indo-Aryan, resulting in just a nominative–oblique inflectional distinction. The New Indo-Aryan languages innovated case markers and developed new case-marking systems. Like in Old Indo-Aryan, case is systematically used to express semantic differences via differential object marking constructions. However, unlike in Old Indo-Aryan, many of the New Indo-Aryan languages are ergative and all allow for non-nominative subjects, most prominently for experiencer subjects. Objects, on the other hand, can now also be unmarked (nominative), usually participating in differential object marking. The case-marking patterns within New Indo-Aryan and across time have given rise to a number of debates and analyses. The most prominent of these include issues of case alignment and language change, the distribution of ergative vs. accusative vs. nominative case, and discussions of markedness and differential case marking.