Imperfective aspect - НСВ

Perfective aspect - СВ

Imperfective verbs may denote an action as a process that took place in the past, is taking place in the present, or will take place in the future, therefore they may be used in the past, present or future tense.

Он писал письмо. - He wrote a letter.

Presents an action as a process, in its development, evolving in time. The action is named only; the speaker does not care about possible result.

Perfective verbs mostly denote the result of an action that occurred in the past, or will occur in the future. Perfective verbs have no present tense, they are used only in the past and simple future tense.

Он написал письмо. - He wrote a letter (from beginning to end).

Denotes the completion of an action, its result.

Обычно мы кончали работу в 5 часов. - We usually finished our work at 5 o'clock.

Denotes habitual, repeated actions.

Imperfective verbs are usually accompanied by the adverbs часто, редко, всегда, иногда, обычно, долго.

Сегодня мы кончили работу в 5 часов. - Today we finished our work at 5 o'clock.

Denotes a single specific action occurring at a specific moment.

Учитель долго проверял письменные работы учеников. - It took the teacher a long time to check the pupils' written work.

The action continued for some time. The process is emphasized.

Учитель проверил письменные работы учеников. - The teacher checked the pupils' written work.

The result is emphasized. The action is single, not repeated.

Пока шёл дождь, мы смотрели телевизор. - While it rained, we watched TV.

Both actions took place simultaneously.

Он открыл окно, проветрил комнату и закрыл его. - He opened the window, aired the room and closed it.

One action which produced a result took place after another, which was also completed with a result.

Ребёнок смеялся. - The child laughed.

The imperfective verb denotes the action itself without the indication of its beginning or end.

Ребёнок засмеялся. - The child began to laugh.

The perfective verb with the prefix за- denotes an initial point of action. The prefix за- with the meaning of the beginning of an action is generally added to verbs denoting noises, movement and light effects.