The Russian verb has 3 tenses and 2 aspects. Aspects describe different qualities of an action—it is either acting (the process of doing something—Imperfective) or the result of an action (after someone has finished doing something—Perfective). Using the Imperfective/Perfective depends only on the intention of the speaker to emphasize different aspects of action in his or her speech.
That’s why the Perfective can be used only in the Past or Future Tense, since the result can be either reached in past, or it will be reached in future. Russians use the Imperfective for the Present Tense, since the present is in progress. If you require emphasizing in your speech a necessity of fulfillment or a desire to finish an action and achieve a result—use the Perfective.
Я сделаю это к вечеру!
Мы напишем отчёт через неделю!
Они уже поговорили с клиентом.
If you need to say that the process of doing something is necessary and important, then use the Imperfective!
Я буду заниматься спортом !
Он будет говорить с директором после обеда.
Они уже делали отчёт на прошлой неделе.
The Perfective aspect of Russian verbs is formed from the Imperfective by using different prefixes and suffixes.
Aspects of verbs denote tenses
The Perfective can be used only in the Past or Future Tense, since the result can be reached only in past, and there is still a chance that there will be a result in future. Also please note that verbs in the Perfective aspect in the Future Tense change persons as verbs in the Imperfective aspect in the Present Tense.