1 12. Diachrony and Grammaticalization 1 Steve Nicolle 1. Diachrony in Tense and Aspect Systems Questions about diachronic change as it relates to tense and aspect (TA) range from the general, such as “How do TA systems come about?” and “How do TA systems change over time?” to the more specific, such as “How do individual tense/aspect markers or particular temporal or aspectual distinctions arise?” and “How do tense/aspect markers change over time?” This chapter will primarily be concerned with the more specific questions, but we will begin with a brief summary of how entire TA systems develop and change. To a large extent the morphological typology of a language determines the manner in which tense and/or aspect categories are expressed. Broadly speaking, languages can be characterized as either isolating, agglutinative, or fusional. In isolating languages, most words consist of a single morpheme, and so TA markers (if they occur) tend to be realized as free morphemes. In agglutinative languages, words can be segmented into individual morphemes, and so each tense or aspect marker is typically realized as a distinct bound morpheme. In fusional languages, morphemes are fused together to form portmanteau forms which simultaneously mark a number of different grammatical categories, and so tense or aspect may be expressed, for example, as part of an unsegmentable affix expressing not only tense or aspect but also the person and number of the subject. In agglutinating and fusional languages, TA markers often occupy a required inflectional “slot” and so TA marking is more likely to be obligatory than in isolating languages. Thus, in the isolating language Vietnamese, no temporal or aspectual marking is required in (1) so long as the time reference can be inferred from the context, whereas in the English gloss, temporal or aspectual marking is obligatory: (1) Chi, áy quên