How do I know when to use these tenses: imparfait, passé simple and passé composé?

The imparfait and passé simple work together to narrate the past: imparfait is used to describe actions that took place over a certain period of time in the past, and it is used for description, whereas the passé simple will be used to describe a specific action in the past, an action that lasted a short and defined while, an action more sudden than those described with imparfait. For instance: "la chambre était belle et grande, le soleil éclairait le mirroir, brusquement, le jeune homme entra" in this sentence, the description is done in imparfait, and is interrupted by the passé simple describing a sudden action. Passé composé and imparfait are also used together, but while passé composé is used for completed actions, actions that have occurred a specific number of times, actions that cut into or interrupt situations being described in imparfait or actions that bring a new information, a change in situation or state of mind, imparfait is used for actions that are incomplete, (that give no indication of when they started or ended), actions that occurred regularly, and to describe the backgrounds that are interrupted by passé composé. For instance in this sentence: 'J'aimais le chocolat mais un jour il m'a donné mal au ventre' the appetite for chocolate is described in imparfait, but the stomach ache it caused the narrator on a specific day is explained in passé composé