Both tenses talk about the past, but they answer different questions. The imparfait describes what things were like, the passé composé reports what happened. Most real storytelling uses both, each doing its own job.
1. The core difference: background vs. event
| Il pleuvait. | It was raining. (ongoing condition, imparfait) |
| Il a plu toute la journée. | It rained all day. (a complete, bounded event, passé composé) |
2. The classic pattern: imparfait sets the scene
The imparfait describes what was already going on, the passé composé is the specific thing that interrupted it.
| Je dormais quand le téléphone a sonné. | I was sleeping when the phone rang. |
| Il faisait beau quand nous sommes arrivés. | It was nice out when we arrived. |
3. A sequence of actions: passé composé
When you're listing what happened, one thing after another, each one is a completed event.
| Je me suis levé, j'ai pris mon café, et je suis parti. | I got up, had my coffee, and left. |
4. A repeated routine: imparfait
| Je me levais toujours à sept heures. | I always used to get up at seven. |
5. Quick reference
| imparfait | description, background, ongoing or repeated, no clear end |
| passé composé | a specific, completed event, or a sequence of them |