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.
Old and Middle Japanese are the pre-modern periods of the attested history of the Japanese language. Old Japanese (OJ) is largely the language of the 8th century, with a modest, but still significant number of written sources, most of which is poetry. Middle Japanese is divided into two distinct periods, Early Middle Japanese (EMJ, 800–1200) and Late Middle Japanese (LMJ, 1200–1600). EMJ saw most of the significant sound changes that took place in the language, as well as profound influence from Chinese, whereas most grammatical changes took place between the end of EMJ and the end of LMJ. By the end of LMJ, the Japanese language had reached a form that is not significantly different from present-day Japanese. OJ phonology was simple, both in terms of phoneme inventory and syllable structure, with a total of only 88 different syllables. In EMJ, the language became quantity sensitive, with the introduction of a long versus short syllables. OJ and EMJ had obligatory verb inflection for a number of modal and syntactic categories (including an important distinction between a conclusive and an (ad)nominalizing form), whereas the expression of aspect and tense was optional. Through late EMJ and LMJ this system changed completely to one without nominalizing inflection, but obligatory inflection for tense. The morphological pronominal system of OJ was lost in EMJ, which developed a range of lexical and lexically based terms of speaker and hearer reference. OJ had a two-way (speaker–nonspeaker) demonstrative system, which in EMJ was replaced by a three-way (proximal–mesial–distal) system. OJ had a system of differential object marking, based on specificity, as well as a word order rule that placed accusative marked objects before most subjects; both of these features were lost in EMJ. OJ and EMJ had genitive subject marking in subordinate clauses and in focused, interrogative and exclamative main clauses, but no case marking of subjects in declarative, optative, or imperative main clauses and no nominative marker. Through LMJ genitive subject marking was gradually circumscribed and a nominative case particle was acquired which could mark subjects in all types of clauses. OJ had a well-developed system of complex predicates, in which two verbs jointly formed the predicate of a single clause, which is the source of the LMJ and NJ (Modern Japanese) verb–verb compound complex predicates. OJ and EMJ also had mono-clausal focus constructions that functionally were similar to clefts in English; these constructions were lost in LMJ.