How do I remember when to use the perfect or the imperfect tense?

In French the imperfect tense is most often used when referring to a habitual action in the past, something that you used to do frequently, and to describe something that happened over an extended period of time. Whereas the perfect tense is used to describe a completed or one off action in the past. For example : L'année dernière je suis allé(e) au cinema pour fêter mon anniversaire. Or L'année dernière j'allais au cinema tous les vendredi soirs. The difference in use can be seen here - in the first example of the perfect tense, Last year I went to the cinema to celebrate my birthday - a completed and one off action , whereas with the example of the imperfect tense , Last year I used to go to the cinema every friday evening- a repeated or habitual past action.