Both imperfective and perfective verbs have past imperfect. The aspectual opposition between imperfective and perfective verbs remains semantic in most verbal forms. Imperatives are usually expressed using perfective verbs if positive and imperfective verbs if negative. Verbal roots were inherently either imperfective or perfective. Verbs exhibit imperfective or perfective aspect, with a moderately complex tense system. Verbs are of imperfective or perfective aspect, often occurring in pairs. Bulgarian has separate inflections for aorist (past imperfective) and general perfective. Ukrainian verbs come in aspect pairs&colon; perfective, and imperfective. There are two main aspectual distinctions in Kashmiri, perfective and imperfective. This distinction is actually one of perfective vs. imperfective aspect. Another important distinction in Chewa is between perfective and imperfective aspect. This is a rough equivalent of the perfective vs. imperfective aspect opposition. Usage of the perfective aspect follows the same pattern as the imperfective aspect. There is no distinction in the future between perfective and imperfective aspect. Like other Samoyedic languages, Nganasan has the opposition of perfective and imperfective verbs.