Der Verbalaspekt: eine Einführung

Almost every Russian verb comes in two versions, an imperfective and a perfective, and you have to choose one every time you speak about the past or the future. English expresses some of the same ideas with "was doing" versus "did" or "have done", but Russian builds the choice into the verb itself.

A quick test

Ask: is the point that the job got done? Вчера́ я написа́л письмо́ ба́бушке: the letter is finished and can be posted. Вчера́ я писа́л письмо́: I spent time on it, and maybe it is not finished. Habits are imperfective: Ка́ждое у́тро я де́лаю заря́дку, "Every morning I do my exercises".

How the pairs are built

• A prefix makes the perfective: чита́ть → прочита́ть, писа́ть → написа́ть, де́лать → сде́лать, гото́вить → пригото́вить. The prefix varies, so learn each pair as a unit.
• A change of suffix: реша́ть ↔ реши́ть, отвеча́ть ↔ отве́тить, покупа́ть ↔ купи́ть.
• A different word altogether: говори́ть ↔ сказа́ть, брать ↔ взять.

The mistake English speakers make most is reaching for the perfective whenever English uses the simple past. "I read this book last year" can be Я чита́л э́ту кни́гу if you just mean you have read it at some point. At this level, aim to recognise the pairs and use the perfective for finished one-off results.