What is the difference between the imperfect and perfect tense in French?

The perfect tense in French is used to describe completed events in the past, i.e. actions that are completely finished. For example, if you wanted to say, 'she played football yesterday', you would use the perfect tense to say, 'elle a joué au football hier'. However, the imperfect tense is used to describe events that are not completely finished, actions that used to happen regularly in the past, or to describe a background action that is happening in parallel to the main action. For example, if you wanted to say, "When I was younger, I used to play tennis", you would use the imperfect tense to say, "Quand j'étais petite, je jouais au tennis".