Perfective aspect

The perfective aspect (совершенный вид глагола) is used when the action is complete, we see the outcome of the action.

Use the perfective:

1. to show the desire or intention to do something.
2. to describe a sequence of actions.
3. when the action is happend once.

How to built:

1. More often, the perfective aspect is formed from the imperfective aspect with a special prefix.
Exmaple:
~ "написать" is the perfective of "писать" ('to write'), ~ "посмотреть" is the perfective of смотреть ('to watch').
For such verbs, their prefix for the imperfective aspect will often be given in dictionaries.
These verbs are easy to learn in pairs (imperfective - perfective forms) as their conjugation in both aspects is the same.

NOTE: You should be careful about prefixes, as they can not only change the aspect, but also a meaning of a verb.

Example:

говорить = to talk (imperfective) - отговорить = to dissuade (perfective) = отговаривать to dissuade (imperfective)

2. verbs with "-ыва" suffix without "- ыв"
3. Some verbs have completely different pairs of aspects.

Example: говорить (imperfective) and сказать (perfective) = to say, speak, tell
брать (imperfective) and взять (perfective) = to take, get, obtain