What difference is there in saying, "I drank the glass of milk" verses saying "I drank up the glass of milk"?. When you say "drank up" you imply that you finished drinking the entire glass of milk. If you simply say "I drank" then we know you drank from the glass, but we don't necessarily know if you finished off the entire glass or not.
Typically in English we don't care whether or not an action in the past has been completed. If you say "I read a book", it could be that you read the whole book, but it could also mean you read through some of it but never finished it (depending on context).
Russian verbs make a distinction between a completed action and a general or uncompleted one. In fact, for just about every past tense English verb there are two different Russian verbs: one that specifies the past action was completed while the other doesn't.
The Russian verbs that imply that an action was completed are called perfective verbs. The ones that don't specify completion are called imperfective (in this instance think of perfect as meaning complete, rather than without error).
Let's break things down a bit more...
Perfective verbs are used for:
Imperfect verbs are used for:
Using Imperfective verbs in the past
I worked/I was working
Here we're not focused on whether the work was finished.
я работал каждый день
I worked every day
Я работал два часа
I worked for two hours
A closer look at verbal aspect:
Perfective verbs are not used in the present tense
Both perfective and imperfective verbs can be used in the past and future tenses. However only imperfective verbs can be used in the present tense.
This makes sense if you think about it. If an action is occurring right now it can't be completed yet. Therefore the perfective aspect doesn't make much sense when describing a present action.
Verbal aspect forms
There are four basic patterns for the forms of perfective and imperfective verbs:
One of the most common ways we get a perfective verb is by adding a prefix to an imperfective verb.
Unfortunately there's no sure fire way to predict which prefix you'll need to turn an imperfective verb into a perfective one.
If a prefix is a something you place at the beginning of a word, an infix is something you put in the middle of a word.
3) Changes in stem/ending
Sometimes an imperfective verb is changed into a perfective one by changing the ending to the stem of the word.
Again, this change isn't something you can reliably predict. You simply have to learn the verbs as pairs and get comfortable with each of them.
Sometimes the pairing of perfective and imperfective verbs doesn't fall into either of the previous three categories, so we put them into a forth "other" category. Verbs in this group can differ widely in the changes between the two aspects.
Tips for learning verb aspect
The concept of verbal aspect can take a little while to get used to. We just don't have much of an equivalent in the English language. When you throw in the fact that the perfective and imperfective verbs can't easily be predicted by any sort of rule or overarching pattern, it all feels even more difficult.
But there's no need to despair. The more you use verbal aspect the more natural it will feel. The best approach to learning perfective and imperfective verbs is simple: take it one step at a time.
From here on out do your best to learn Russian verbs in pairs (imperfective/perfective). Practice using both forms with your Russian teacher or language partner.
After a little while it will be easier to remember the pairs and you will start to intuitively feel the difference between the imperfective and perfective aspects.