(By Östen Dahl and Viveka Velupillai): "The distinction between imperfective and perfective plays an important role in many verb systems and is commonly signalled by morphological means (rather than being expressed periphrastically). A particularly straightforward case is found in Rendille (East Cushitic; Kenya). Nonstative verbs in Rendille distinguish two basic forms, one which normally ends in -a and one which normally ends in -e, as illustrated by the examples in (1)... ❡
The imperfective form in -a is used for reference to the present and the future but also for ongoing and habitual events in the past, as indicated by the translations. The perfective form in -e is basically restricted to single completed events in the past (with some vacillation for past habitual contexts). In most other languages with an imperfective/perfective distinction, this pattern is obscured by interaction with other tense/aspect grams, but the basic opposition between one form (or set of forms) which is used exclusively or almost exclusively for single completed events in the past and another form (or set of forms) which is used for everything else is characteristic of the distinction. ❡
To be interpreted as a perfective, we demand that a form should be the default way of referring to a completed event in the language in question. In many languages, there are forms or constructions that are used of completed events but only if some additional nuance of meaning is intended, for instance if emphasis is put on the result being complete or affecting the object totally. Such strong perfectives (“conclusives” in Dahl (1985) and “completives” in Bybee et al. (1994)) exhibit relatively large variation cross-linguistically. They are often called “perfectives” in grammars but are not counted as such here..."