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

The imperfect tense (l'imparfait) and the perfect tense (le passe compose) are both past tenses but are used differently.The imperfect tense is used to express 1) continuous actions and events that happened over a long period of time eg J'étais à l'école depuis... (I was at school since...)2) habitual/repeated actions that somebody/something 'used to' do eg Elle jouait au foot chaque lundi (She used to play football every Monday)3) ongoing characteristics or qualities of somebody/something eg La mer était bleue (The sea was blue) The perfect tense is used to express completed, specific events or actions eg Hier, j'ai acheté un livre (Yesterday I bought a book)