may be described as perfective in one clause, and then imperfective in the next. For example, "John read that book yesterday; while he was reading it, the postman came," where the two forms of 'to read' refer to the same thing. In 'John read that book yesterday', however, John's reading is presented as a complete event, without further subdivision into successive temporal phases; while in 'while he was reading it', this event is opened up, so that the speaker is now in the middle of the situation of John's reading, as it is in the middle of this reading that the postman arrives. The perfective and imperfective need not occur together in the same utterance; indeed they more often do not. However, it is difficult to describe them in English without an explicit contrast like "John was reading when I entered." Combination The two aspects may be combined on the same verb in a few languages, for perfective imperfectives and imperfective perfectives. Georgian and Bulgarian, for example, have parallel perfective-imperfective and aorist-imperfect forms, the latter restricted to the past tense. In Bulgarian, there are parallel perfective and imperfective stems; aorist and imperfect suffixes are typically added to the perfective and imperfective stems, respectively, but the opposite can occur. For example, an imperfect perfective is used in Bulgarian for a simple action that is repeated or habitual: Here each sitting is an unanalyzed whole, a simple event, so the perfective root of the verb 'sat' is used. However, the clause as a whole describes an ongoing event conceived of as having internal structure, so the imperfective suffix -eshe is added. Without the suffix, the clause would read simply as In the evening he sat on the veranda.'' References Grammatical aspects