How do you know when to use the imperfect tense?

The imperfect tense is used to described incomplete, continuous actions in the past. Examples of this would be "I was studying", or "I used to play football", because they do not refer to one specific action. Therefore, in French, they become "J'étudiais" and "Je jouais du foot". The imperfect can also be used to describe a continuous state of being or feeling, for example, "I was happy" becomes "J'étais heureux(se)", whereas the passé composé would be used to describe a change in feeling, or a new feeling: "Suddenly, I was hungry" becomes "Soudainement, j'ai eu faim". The imperfect is also used to describe actions within the past, almost like a background tense, whilst the passé composé is used to describe new, completed actions: "I was washing the dishes when my mum came into the kitchen" becomes "Je faisais la vaisselle quand ma mère est venue dans la cuisine". So, to sum up, the imperfect tense is used to describe ongoing, or continuous actions in the past.