Imperfective aspect

The imperfective aspect (несовершенный вид глагола) is used when the action is incomplete, it describes a process. This characteristic does not depend on time of the verb, so it exists equally in past, present and future tenses.

Use the imperfective aspect when:
1. an action is in the process
2. an action is repeated or done regularly
3. you describe a skill using special verbs: уметь (= can), научиться (= to learn), привыкнуть (= to get used to) etc.
5. you describe the beggining, process of the end of an action using special verbs: начать (= to start), закончить (= to finish), продолжить (= to continue), завершить (= to finish), перестать (= to stop) etc.

Imperfective verbs:
1. with "-ыва" suffix
2. there are a few verbs that are essentialy imperfective and do not exist in perfective aspect such as быть (= 'to be').

Imperfective aspect of the verbs is usually mentioned in dictionaries.