Verbal aspect: every verb has two faces
Imperfective is process, repetition or bare fact; perfective is result and completion. Aspect is lexical, not a grammatical ending.
Aspect is not a tense and it is not an inflection. Almost every Russian verb exists as a pair of separate vocabulary items — one imperfective (НСВ) and one perfective (СВ): де́лать — сде́лать, писа́ть — написа́ть, чита́ть — прочита́ть, говори́ть — сказа́ть. You learn them the way you learn the gender of a noun: as part of the word. There is no ending you can attach to a verb to change its aspect, and no rule that reliably predicts the partner, although prefixes do most of the work.
The four standard labels are worth learning explicitly, because they are the questions you ask yourself when choosing. Факт, the bare fact that something happened: Ты чита́л э́ту статью́? — did you read it at all? Проце́сс, the action under way: Когда́ ты пришёл, я чита́л. Повторе́ние, the same action repeated: Ка́ждый ве́чер он чита́л газе́ту. And on the perfective side, Результа́т: Я прочита́л э́ту статью́ — I have read it, I know what is in it. To that add завершённость, simple completion: Я написа́л письмо́ — the letter is written and now exists.
The tense system follows from this. The present tense exists only for the imperfective. A perfective verb has no present at all: the form я прочита́ю looks like a present but is a future. So the imperfective has a present, a past and a compound future; the perfective has a past and a simple future. Both aspects have exactly the same four past forms — чита́л, чита́ла, чита́ло, чита́ли — and the difference between them is entirely one of meaning.
Choosing between them is a matter of what you are reporting. Use the perfective for an outcome, a single completed act, a change of state, or a sequence of completed steps: Я вста́л, умы́лся и вы́шел. Use the imperfective for process, habit, repetition and general facts: Я встава́л в семь ка́ждый день. Two further habits are worth acquiring early. Negation normally takes the imperfective, because you are describing a state of affairs rather than a failed completion: Я не чита́л э́той статьи́. And after a modal the choice changes the meaning of the whole sentence: Я хочу́ чита́ть is 'I want to be reading' or 'I want to read in general', while Я хочу́ прочита́ть э́ту кни́гу is 'I want to get through this book'.
Examples
- Я чита́л э́ту статью́. I read this article at some point. — imperfective: the bare fact Listen in Russian: Slow in Russian: Я чита́л э́ту статью́.
- Я прочита́л э́ту статью́. I read this article and finished it. — perfective: the result Listen in Russian: Slow in Russian: Я прочита́л э́ту статью́.
- Когда́ ты пришёл, я чита́л. When you came in I was reading. — imperfective for the process under way Listen in Russian: Slow in Russian: Когда́ ты пришёл, я чита́л.
- Ка́ждый ве́чер он чита́л газе́ту. Every evening he read the newspaper. — imperfective for repetition Listen in Russian: Slow in Russian: Ка́ждый ве́чер он чита́л газе́ту.
- Он написа́л письмо́ и отпра́вил его́. He wrote the letter and sent it. — perfective for a chain of completed acts Listen in Russian: Slow in Russian: Он написа́л письмо́ и отпра́вил его́.
- Я не чита́л э́той статьи́. I haven't read that article. — negation normally prefers the imperfective Listen in Russian: Slow in Russian: Я не чита́л э́той статьи́.