Verb aspect formation. (#0022)

1. It's almost impossible in Russian to predict how a perfective verb is formed. Most dictionaries will list both forms in the same entry. Some perfectives are formed by adding a prefix to the imperfective form:

   imperfective	perfective
   делать	сделать
   строить	построить

2. Some aspect pairs are formed by extending the stem of the imperfective form with an infix:

   imperfective	perfective
   показывать	показать

3. If one verb of the pair is conjugation I, and the other is conjugation II, most often the conjugation II verb will be perfective:

   imperfective	perfective
   кончать	кончить
   отвечать	ответить

4. Some aspect pairs are formed from two different stems:

   imperfective	perfective
   говорить	сказать