When do you use the passé composé or the imparfait and how do you form these two tenses?

The passé composé is used to describe a completed action in the past.To form the passé composé you will need the subject, the auxiliary and the past participle of the verb:e.g. J’ai joué (I played) – Je = the subject, ai = the auxillary, joué = the past participle (of jouer - to play). The auxiliary is formed from the present tense of either avoir (to have) or être (to be). Most verbs will take avoir (including the past tense of avoir and être), but all pronominal and reflex verbs will take être. Movement verbs will also take être. To conjugate the past participle, most verbs will follow the following rules: -er verbs = é (jouer = joué), -ir verbs = i (chosir = choisi), -re verbs = u (répondre = répondu)When a verb does take être, you will also have to make sure that the past participle agrees with the subject, e.g.:Elles sont allées – They (female plural) went
The imparfait has several uses in French, for example:-      Description in the past (appearance, age, state of mind, etc.) Les maisons étaient toutes les mêmes dans la rue. (The houses in the street were all the same)-      Interrupted actions where the English was / were is often used Je lisais mon journal quand le téléphone a sonné. (I was reading my newspaper when the telephone rang)-      Habitual actions, where the English used to / would is often used Le lundi il allait à sa leçon de piano (He used to go to his piano lessons on Mondays) -      Non-completed actionsIl étudait le français depuis 10 ans. (he has been studying French for 10 years)To form the imparfait: -      For the stem: Take the nous form of the present tense of the verb and minus the -ons. E.g. jouons = jou-This is regular for all verbs except être, whose stem is ét--      For the endings: Je = -ais (jouais), Tu = -ais (jouais), Il/Elle/On = -ait (jouait), Nous = -ions (jouions), Vous = -iez (jouiez), Ils/Elles = -aient (jouaient)