What is the difference between the perfect and imperfect tense?

The perfect tense is use to express actions which heve certainly been completed. In some cases it is used because it only happened once.For instance "J'ai passé mes vacances scolaires à Lyon avec ma famille"The action is complete, you have now returned from the holiday and so the perfect tense is required.If you wanted to say that for the last few years this has been the case, you might use 'Depuis...' which is then followed by the present tense, requiring neither perfect nor imperfect tense.So in this case; "Depuis plusieurs années ma famille et moi passons mes vacances scolairesà Lyon" Whereas the imperfect tense is used when refering to a continous past action. The actions happen more than once.For instance, your friend plays tennis every saturday so... "Il jouait au tennis plus tôt, comme chaque samedi" Because this action is something he does every week, the imperfect tense is required. The imperfect is often used to express general reactions, by using the phrase "c'était"