How do you know when to use the perfect tense or the imperfect tense?

Both the imperfect and the perfect tense are used to talk about something that happened in the past. However, the imperfect tense tends to refer to things that happened over a period of time in the past. For example, I was eating my dinner = 'je mangeait mon dîner'. However, the perfect tense is used to talk about things which happened as a one off or happened immediately. For example, she lost her bag 'elle a perdu son sac'. 

Sometimes both tenses can appear in one sentence. For example, I was washing the dishes when she entered the room = 'elle faisait la vaiselle quand elle est entrée'. Both of these tenses are very dependant on the context of the action.