When should I use the perfect tense and the imperfect tense?

The perfect tense is used to describe a completed action in the past. (Sometimes, time connectives are used as an indication that an action is complete). e.g. Hier soir, j'ai fait mes devoirs - Yesterday evening, I did my homework. The imperfect tense is used to describe ongoing actions in the past that had a duration, or habitual actions in the past (i.e. things that 'used to' happen) e.g. Quand j'étais plus jeune, je jouais au foot - When I was younger, I used to play football. These two tenses can be used together to describe an interrupted action in the past. For example, 'I was eating my dinner when the phone rang'. The ongoing action of eating (imperfect tense) is interrupted by the completed action of the phone that rang (perfect tense). Therefore it translates as: Je mangeais mon dîner quand le téléphone a sonné.