Passé composé or imparfait: choosing a past tense

English has one everyday past tense. French has two, and they are not interchangeable - the same sentence in each says something different about what happened. This is the choice French learners hesitate over for years, and it is more learnable than it looks.

Passé composé: what happened

The passé composé is for things that happened - single, finished events that move a story forward. J'ai mangé, elle est partie, nous avons décidé. If you could put it on a timeline as a point, it takes the passé composé.

Imparfait: how things were

The imparfait is for how things were - background, habits, weather, age, feelings, and anything that was going on when something else interrupted it. Il faisait froid, je travaillais, elle avait vingt ans. It is the scenery, not the action.

Both in one sentence

Most real sentences use both, and that is the clearest way to feel the difference. Je dormais quand le téléphone a sonné - I was sleeping, the phone rang. The imparfait sets the scene, the passé composé breaks it. Swap them and the sentence stops making sense.

The être trap

One warning worth having early: the passé composé is built with avoir for most verbs but être for a small group of movement verbs and all reflexives, and with être the participle agrees with the subject - elle est allée, not elle est allé. That agreement is the mistake that survives longest.