When should you use the imperfect tense and how is it different to the perfect tense?

The perfect tense is for actions that happened once in the past (e.g. I did) whereas the imperfect is for repeated actions (e.g. I was doing). So for something you used to do , you would use the imperfect tense. You should also use the imperfect for descriptions like weather (il faisait beau). If an action is interrupted by something (e.g. I was eating when you called) you should use imperfect for the action that is interrupted and perfect for the action that interrupted.