How do I know whether to use the imperfect tense or the passé composé?

To begin with, it is important to understand that it is complicated telling the difference between these two tenses because in English, they are not distinct when translated so you can’t always rely on how we say it in English to determine the tense. We need to understand what imperfect and passé composé actually mean, because the names of the tenses can sometimes stump you. Simply put, without using any terminology that makes it sound more difficult than it is – the imperfect tense is for something occurring in the past that is ongoing, background information, a habit etc. and the passé composé is a completed action in the past, a specific event, something that we can clearly note the start and the end. I find it easier to explain the difference with examples: ‘I was playing football’, or ‘I played football when I was young’ uses the imperfect  (je jouais ...); whereas ‘I played football on Friday’ uses the passé composé (j’ai joué ...) as it is a one off, complete event.I like to picture my tenses on a timeline, and if I want to plot the passé composé, it is a dot, a completed action that has a definite start and end. To plot the imperfect it is a wiggly line in the past, something ongoing (I would draw this on the whiteboard).