when do you use the passé compose

When to use the passé composé and when to use the imparfait is a source of a lot of confusion since French uses the tenses differently from English.

I always think about this in terms of describing a theater performance. The imparfait is used to describe what the stage and the scene looks like whereas the passé composé describes the action itself taking place on the stage. For example:

Il pleuvait et il faisait sombre dehors. Le voisin promenait son chien. Juste à ce moment-là une voiture est passé devant chez moi.

It was raining and dark outside. The neighbour was walking his dog. -- Description of the "setting", hence imparfait.
Juste à ce moment-là une voiture est passé devant chez moi. -- Description of the "action", hence passé composé.

There are several other uses of the imparfait (e.g. in conditional sentences with "si" and reported speech, etc.) but these are less confusing because they follow a strict grammatical rule.

I hope that helps to clarify the issue a bit.

-- Chris (not a native speaker).

Bonsoir Irene, Le passé composé is the conversational past tense (this is the most common use of it). There are other nuances but I will not present those on this response. You will run across them as you go along. This link is from the Progress with Lawless French Library and it explains the passé composé in detail along with examples: https://kwiziq.learnfrenchwithalexa.com/my-languages/french/glossary/44 There is a whole page of links to lessons on the passé composé. I do suggest that you get a good basic understanding of le passé composé and l'imparfait before attempting any other levels. But as noted previously, the most common use of le passé composé is in conversation in discussing the past, i.e. j'ai mangé --> I ate, I have eaten, etc. Once you have this mastered, several of the other past tenses fall into place a lot easier. J'espère que ma réponse vous aiderait. Bonne chance et bonne continuation dans vos études en français, la langue de Molière et qui a été utilisée par le monde français depuis l’époque d’Hugues Capet Ron (un locuteur non natif )

To expand on this a bit:

If you are describing something in the past that is merely a description of something which happened usually or over some period of time, this also qualifies as "backdrop" and requires the imparfait.

For example:

Quand j'étais enfant,  je jouais souvent avec ma balle préferée. -- As a child I often played with my favorite ball.

This is a description of something which took place habitually and therefore one uses the imparfait.

-- Chris (not a native speaker).