when to use the passé simple or the imparfait?

The imparfait and the passé simple are two past tenses in French but both are used differently.The imparfait is used to: describe a past action in which the time is not determined such as: "il lavait le toit" (he was washing the roof) : time of the action is not determined.a repeated action in time "tous les samedis il allait nager" (every Saturday he went swimming).a description: "les fleurs blanchissaient avec le temps" (flowers whitened with time)The verb ending of the imparfait is:-ais; -ais ; -ait ; -iez; -ions; -aient for all verbs of the 3 groups Important: for verbs of the 1st and 3rd group the "I" for the first and second pronom of the plural must be maintainedexemple "nous riions"
The passé simple is used to:a past action that was determined in time (as opposed to the imparfait): "il mangeait quand elle lui dit..." (he was eating when she told him..)a sudden action: "il mangeait quand soudain elle lui cria..." (he was eating when all of a sudden she screamed...)For the verbal ending of the passé simple: there a 4 different depending on what groupe of verbs the verb is in.  1st group and the verb aller: -a : -ai, -as, -a, -âmes, -âtes, -èrent.  2nd and third group:  -i : -is, -is, -it, -îmes, -îtes, -irent.   3rd group: -u : -us, -us, -ut, -ûmes, -ûtes, -urent.  3rd group:  -in : -ins, -ins, -int, -înmes, -întes, -inrent.