One of the most difficult things in Russian is the usage of correct verbal aspect. It's also the most common mistake which foreigners make in writing. When a person writes a story about himself, his experience and events, he can easily mix up verbal aspects. The meaning of a text would be clear but its grammar would be misrepresented and sound unnaturally. Sometimes it even concerns a meaning.
Firstly, you should memorize that a perfective verb doesn't have present tense. Only past and future tenses. It can be easily explained. "Совершенный вид" [savirshenyy vid] (perfective aspect) - as you can understand fr om the name - describes what has been already done or will be done, i.e. the process which has been completed or is intended to be completed. "Несовершенный вид" [nisovirshenyy vid] (imperfective aspect) describes what happens occasionally or lasts without any sign of process end and what has not been happened at all or due to continuality.
Let's look at the verb "делать" [d'elat'] - do.
Imperfective aspect (Н.В.): Я делал домашнюю работу. - I have been doing my homework. (We say that we have been in the process but we don't say that we have finished our homework). Я делаю домашнюю работу. - I am doing my homework. (I'm in the process but the work has not been done yet). Я буду делать домашнюю работу. - I will be doing my homework. (I will be in the process but I don't say whether the work will be finished or not).
Perfective aspect (С.В.): Я сделал домашнюю работу. - I have done my homework. (The work is finished). Я сделаю домашнюю работу. - I will do my homework. (I will finish my work).
Imperfective verbs are formed fr om perfective ones. There are some verbal aspect pairs which I've divided into groups according to their manner of derivation.
Suffixes спросить - спрашивать [sprasit' - sprash'ivat'] - to ask дать - давать [dat' - davat'] - to give одеть - одевать [ad'et' - adivat'] - to dress
Prefixes читать - прочитать [chitat' - prachitat'] - to read писать - написать [pisat' - napisat'] - to write готовить - приготовить [gatovit' - prigatovit'] - to cook/prepare
As you have not memorized verbal pairs yet and don't know what verbal aspect to use, just ask yourself "what action do I have in mind? completed or not?"
Let's try to consider the typical mistakes together.
"Я пришел в магазин, где женщина продала книги" - I have come to the store, where a woman had sold books.
The author says that he has obtained a result (has come to the store), that's why the choice of the perfective aspect "пришел" [prishyol] (have come) is correct. The second verb has two aspects: perfective "продала" [pradala] (had sold) and imperfective "продавала" [pradavala] (had been selling). It is understood that the woman sells books for a long time up to now. This action is incomplete. So it's correct to say: "Я пришел в магазин, где женщина продавала книги" - I have come to the store, where a woman had been selling books.
"Вчера я решал, что брошу курить" - Yesterday I was deciding I would quit smoking.
I decided I would quit smoking = I decided I would be done with smoking. We are talking about a result, about completed process. That's why "брошу" [broshu] (would quit) is correct form. We are talking about the decision which is made, the process of thinking has been completed with the decision. Decision is a result. That's why it is correct to say: "Вчера я решил, что брошу курить" - Yesterday I decided I would quit smoking.
"Я посмотрю телевизор каждый день" - I will watch TV every day.
In this case the perfective verb "посмотреть" [pasmatr'et'] (watch) is used incorrectly. We are talking about the action which is repeated on a regular basis. The correct sentence: "Я смотрю телевизор каждый день" - I watch TV every day.
Thus, incompleted action, process - imperfective aspect; repeated action - imperfective aspect; completed action, result - perfective aspect.
каждый [kazhdyy] - every/each, часто [chasta] - often, редко [retka] - seldom, иногда [inagda] - sometimes, обычно [abychna] - usually, регулярно [rigul'yarna] - regularly - imperfective aspect.
утром [utram] - in the morning, вечером [v'echiram] - in the evening, сегодня [sivodn'ya] - today, назад [nazat] - ago (e.g. two days ago), в понедельник [v panid'el'nik] - on Monday, уже [uzh'e] - already, еще не [ishch'yo ne] - not yet - perfective aspect.