Once I heard a joke: you can say that you ate a pie in any language, but only in Russians you may let other people know whether you left anything for them. This joke express the idea of distinction between perfetive and imperfective aspects pretty accurately. If there's nothing left in the dish, you can assume that it was a perfect pie and it was eaten entirely. Here you have to use a perfective verb “съесть” [syest'], since the action was completed. However, this is not the only meaning of the perfective aspect, and the more you think about it, the more confusing it gets. Let's try to make a complex thing simple.
The first thing about the verbal aspects is: every verb in Russian has an aspect. It means that every verb is either imperfective or perfective. There's no verb in Russian that doesn't have an aspect. There's no verb in Russian that could be both perfective and impervective (well, actually, there are a few perfective verbs that are absolutely like their imperfective counterparts, but it is not important here).
The distinctive feature of all perfective verbs is that they represent an action as a whole, and this action exists in the limited space of time. The idea of the limit is quite abstract here. It could be the idea of getting something done (съесть - to eat something to the last crumb, прочитать - to read a book to the rear cover, вымыть – to clean or wash something entirely etc). It also could be the idea of stopping doing something without getting any noticeable result. You have stopped doing something, thus, you are not doing it anymore, therefore, use a perfective verb (Я поела суп– I have being eating a soup for some time and then stopped eating and started doing something else; я почитала книгу – I have being reading a book for some time, but I'm not done with the book, I have just switched to another activity; я помыла руки – I just state that I have washed my hands, but I can not give any guarantees that they are absolutely clean). Also, the idea of the limit may refer to the beggining, and there are a bunch of perfective verbs that represent the starting of doing something (Она запела – she started singing; они побежали – they started running; он приуныл – he became sad). In every example that I gave above, the action is represented as something whole that happened (or will happen) once, non-repeatedly.
Imperfective verbs, on the other hand, represent an action as something that happens regular or as a process. Russian imperfective aspect is a mix of English indefinite and continuous tenses. Thus, он пьёт пиво could mean that either he is drinking beer at this very moment (hey, leave me some!) or he normally doesn't refuse the idea of getting some beer in proper circumstances. The exact meaning should be clarified from the context.
Why do Russians (almost) never get confused when choosing a right aspect? For the same reason why native English speakers have (mostly) no difficulties with choosing between definite and indefinite articles. When you hear your native speech from the earliest childhood, you just use the grammar intuitively. Start to think about it, and you won't be able to say a word.
Does it mean that there's no way for non-native speakers to master the aspect usage? No, it doesn't. Like with any other grammar rule, practice will make you perfect. But before your brain gets enough statistics through listening, reading, writing and speaking in Russian that the right aspect just comes to the end of your tongue automatically, you have to perform some analysis for each verb you use in your text (or at least, when you feel unsure). Is an action you describe with a verb a process or is it something that happens on a regular basis? If yes, then use imperfective. Is it something that happened once (or will happen just once), it has its beginning and/or its end? If yes, then use perfective. It should work in most cases.
Photo by Krissy Venosdale