How do you choose between using the imperfect and the perfect tenses?

In general, the main thing to remember with the perfect tense is that it is used with a completed action in the past. The imperfect tense can be used for several purposes: to describe a person's character or appearance in the past, convey that actions were repeated in the past or that an action was not completed. For example, phrases such as 'used to', 'always/often/sometimes' require the imperfect: "J'allais au cinéma chaque semaine" (I used to go to the cinema every week) versus "Je suis allé(e) au cinéma il y une semaine" (I went to the cinema a week ago). There are times when the perfect and imperfect are used together. For example, if an action that was being carried out in the past (imperfect) is interrupted by another event (perfect): "Il preparait le déjeuner quand sa soeur l'a appellé".