Which tense should I use: the imperfect or the passé composé?

It's helpful to think about the passé composé tense as the tense you would use when the action you are talking about happened at a specific point in time. You would use the imperfect tense to describe actions that are either: A) actions you did regularly in the past (if you can use the phrase "used to" then it is imperfect) B) background actions that were happening when something more important or notable happened. C) actions were there is no definite indication of whether the action has been completed or not (as a rule of thumb, if you can say "I was -ing" then it is imperfect) We can look at some examples to illustrate these three cases. A). "Last friday, I played football" vs "I used to play football every friday". The first sentence talks about something that happened at a specific moment in time so we'd use the passé composé. The second talks about an action that happened regularly an uncounted amount of times, so we'd use the imperfect. B). "I was watching tv when the doorbell rang". The first part is descriptive 'setting the scene' talk, whereas the second part indicates an event that happened at a specific moment in time, not something that happened over a more general time period. We'd use the imperfect tense for the first part and the passé composé for the second part. C). "I was reading a book" vs " I read a book". The first sentence does not tell the audience whether you have finished the book, while the second sentence indicates that you have read and finished the book. To help, try to see what fits naturally on the end of such a sentence; if it is followed by 'when + action happened' then it is imperfect, if it is 'when + an indication of when it happened' then it is passé composé.