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.