It is the form of the verb which shows the character of the action from the point of view of its progress or its completion.
1) The perfective Прочитал Сделал 2) Non-perfective Читал Сделал
As there are no such clear-cut distinctions between English verbs, many foreign grammarians are of the opinion that such a category does not exist in the English language. They speak of only tense distinctions and thus find four groups of tenses: 1) Indefinite 2) Continuous, 3) Perfect 4) Perfect continuous
By lexical means they understand the lexical character of the verb. Its a well- known fact that English verbs fall under two classes: terminative (those which denote an action implying a certain limit beyond which it cannot go: to come, to stop, to complete, etc.) durative (those which denote an action which does not imply any limit, that is without an end in view: to study, to carry, to keep, etc.)
On the other hand, our grammarians claim that aspect distinctions in English may be expressed by means of grammatical forms. The most widely spread view on this point is that the category of aspect is expressed through the opposition of continuous/ non- continuous (or common) aspect forms (asks- is asking) in which the Common aspect is the unmarked member, whereas the Continuous aspect is marked, its marker is the combination be + ing.
As to their meaning the marked member (the Continuous aspect) has a specialized clear-cut meaning of an action in its progress, developing at a given moment; the unmarked member (the Common aspect) is very general, it represents an action as merely a state or a fact, therefore the best way of defining it is negatively, that is denoting no duration at all.