How do you know when to use passé composé or imparfait?

When using the past tense, you usually have to choose between passé composé and imparfait. Imparfait is often used for continuous, or habitual actions that have happened but could still be happening. Tous les jour, il achetait son journal au coin de la rue. Everyday, he bought his paper at the street corner. The passé composé expresses a foreground action that took place once and will not happen again. It is not habitual, but punctual. The passé composé is often used to narrate what a character has done in the past. In a way, it has a "foreground" use, whereas the imparfait is rather "background" action. Cette nuit-là, il tomba du lit. That night, he fell off his bed. Here is a general example to illustrate the two usages: Le vent sifflait et le soleil se couchait quand Pierre rentra chez lui. The wind was whistling and the sun was going down when Pierre went home. In this example, the emphasis is on Pierre going home, an action singled in time. The wind whistling and the sun going down where happening as well, but these actions will repeat themselves and are therefore not completed.