Hello Danilo! If you want to know how to use the past tenses in French, take you mother tongue as a start. I guess it would be easier than starting from English since we don't make the same distinction. In French, we usually say (although that's very simplified and not always true) that "imparfait" is for description and simultaneous actions whereas "passé simple/passé composé" are for successive actions. "Passé simple" and "passé composé" are also used to translate the perfect, i.e. an action or an experience considered as finished. With these rules, you won't get it always right, but that's a start. You need to develop a feeling for it. "Passé simple" is not use merely for writing but can also be used oral to sound a bit pedantic. "Passé composé" is used orally but can also be used in writing to give a modern touch. In writing, you should never mixt both tenses as it is considered improper and unelegant. Good luck !

http://apremont.cycle3.free.fr/classe_virtuelle/fichiers/regles/C7.pdf Le passé simple s'emploie dans les récits écrits : contes, romans, récits historiques mais pas à l'oral.

Hi Soufiane. It's more clear for me now. Merci Beaucoup.

The imperfect is used for description, speak of a habit. The action is incomplete: (Quand il faisait froid il n'oubliait pas de mettre ses gants sur la cheminée.) =When it was cold he did not forget to put his gloves on the chimney. Here it is a habit. For " le passé composé" the past tense; the action is accurate and complete. Il a mis ses gants sur la cheminée en rentrant du travail. (He put his gloves on the chimney home from work.)