oryx_and_crakeSome comments on perfective vs. imperfective issue that proved to be a non-trivial one.
In English, it is the usage of the verb that conveys the idea of perfectiveness or imperfectiveness of action.
Compare the usages of the verb to happen:
It has happened. It will happen tomorrow. - completed action
It is happening now. - prolonged action, non-perfective.
Not so in Russian. To convey the meaning of the above-mentioned sentences you will have to use two different verbs:
Это произошло. Это произойдет завтра. - произойти, perfective
Это происходит сейчас. - происходить, imperfective
So, the fate of a Russian verb is, so to say, written on its forehead - you don't have to construct phrases with it to find out whether it is perfective or not. Moreover, if a verb is imperfective it will be imperfective no matter in which phrases and in how many phrases do you put it. The dictionaries normally tell you whether a verb is perfective or imperfective. Most verbs (not all of them though) exist in perfective/imperfective pairs, like the abovementioned произойти/происходить.
UPDATE. Corrected the first English example, thanks everybody for the input.