Almost every intermediate learner asks the same question, and almost every textbook answers it badly. The textbook says: passé composé is for completed actions, imparfait is for ongoing or habitual ones. True, and almost useless in a real conversation, because in the moment you can't tell which one your sentence is.
Here's the more useful version.
The tense isn't in the verb. It's in your camera angle.
French doesn't ask you what happened. It asks you how you're filming it.
- Passé composé is a photograph. One event, framed, finished. J'ai ouvert la porte.
- Imparfait is the panning shot. The scenery, the weather, the mood, the thing that was already happening when the photograph got taken. Il faisait froid.
The same real-world event can take either tense depending on which shot you're describing:
Hier, j'ai travaillé huit heures. (the photograph: a finished block of work)
Hier, je travaillais quand tu as appelé. (the panning shot: background, interrupted)
Nothing changed about yesterday. Only the framing changed.
The mistake almost everyone makes
Learners treat state verbs as if they had a fixed tense. They don't, but they default to the imparfait, because states are usually scenery rather than events.
✕ J'ai été fatigué toute la journée.
✓ J'étais fatigué toute la journée.
"Being tired all day" is scenery, not an event. But push the same verb into a bounded, reactive moment and the passé composé is suddenly right:
✓ Quand j'ai vu la facture, j'ai été choqué.
That's a photograph: one reaction, one moment, done.
How to practise it so it actually sticks
Do not do fill-in-the-blank exercises. They train you to analyse a sentence someone else already built, which is not the skill you need. You need to choose while speaking.
Instead: tell the story of your yesterday out loud, for two minutes, every day for a week. Force yourself to include one interruption ("I was doing X when Y happened"). Record it. The reflex you're building isn't grammatical knowledge. It is the instinct to notice, mid-sentence, whether you're filming scenery or taking a photograph.
That's the shift. After that, the rule stops being a rule and starts being a habit.
Common questions
Is there a trick to choosing between imparfait and passé composé?
Ask whether you are filming the scenery or taking a photograph. Scenery, background and habit take the imparfait. A single finished event takes the passé composé.
Can the same verb take both tenses?
Yes, and that is the point. "J'étais fatigué" describes a state, "j'ai été choqué" reports a reaction at one moment. The verb is identical, the framing is not.
Why do fill-in-the-blank exercises not help much?
They train you to analyse a sentence someone else already built. In conversation you have to choose while speaking, which is a different skill and needs speaking practice.