When to know when to use simple past or imperfect tense?

The imperfect tense is used for an ongoing action, meaning that the action has started but it hasn't finished yet. It is lasting in time, for example: "Je tenais dans mes mains une assiette." ("I was holding a plate in my hands."). In a text written in past tense, it is used to describe the background of the narration such descriptions ("Il pleuvait dehors." / "It was raining outside.") or commentaries made by the narrator. ("Sarah devait être pressée." / "Sarah seemed to be in a rush."). Imperfect can be used both orally or in writing.

Simple past on the other hand is used for actions limited in time and finished: they have a beginning and an ending, i.e: "L'assiette se brisa." (The plate broke."); the plate here is broken, the action is punctual, it occurs only for a brief moment and is finished. This tense is mostly only used for narrative pieces such as novels or short stories. With the imperfect tense, they serve as a basis for the narration. Oftentimes, the action in simple past interrupts the background action in imperfect, i.e: "Je marchais quand le loup m'attaqua." ("I was walking when the wolf attacked me".).