When to use the french imperfect when sometimes the englsh doesn't

You shouldn't think about it in terms of English. In fact, English doesn't usually make the aspect distinction that Spanish, French, Italian, etc. do. The best thing to do, I think, is to understand imperfect vs. preterit; that is, the difference between the two. For example the imperfect tends to refer to continued, habitual, repeated actions in the past, while the latter refers to a specific completed action. After this you should just learn common expressions to help you better understand when it is used in a sentence. Hope this helped!