passe compose is only used when you are talking about something in the past whether it was something you saw or something you did, etc... Imparfait is used when you are talking about something that is repeated, continued, is a habit ( you do it all the time), or an action is incomplete. For example, "Elle a mange a huit heures."
The passé composé consists of either avoir or être + participe passé of the chosen verb. You use it generally for things that happened once or don’t repeat that often. The imparfait is the verb minus its terminaison plus the imparfait terminaison. You use it for things that lasted, or are repeated like habits etc. It is usually used next to passé simple in literature.
