When do I use the imperfect tense and when do I use the perfect tense?

Both the perfect and the imperfect are past tenses. This means that they're both used to describe actions that have already happened.However, there are different uses for each tense. The imperfect is used to describe an ongoing action in the past. Eg- He was talking to his mother on the telephone. --> Il parfait avec sa mère au téléphone. The perfect is used to indicate a sudden action that occurs, usually brief and interrupting another action. Eg - He was talking to his mother on the telephone when he fell over. --> Il parfait avec sa mère au téléphone quand il tomba.