When do you use the passé composé and when the imparfait?
The passé composé reports a completed event that moves the story forward; the imparfait describes the situation it happened in. If you are saying what happened, use the passé composé. If you are describing what things were like, what used to happen, or what was going on when something interrupted, use the imparfait.
Events against background
The passé composé is the plot and the imparfait is the set. In any narrative the imparfait tells you what the world was like and the passé composé tells you what happened in it. A story told entirely in one tense reads as broken to a French speaker even when every individual verb is defensible in isolation.
Il pleuvait quand je suis sorti.
It was raining when I went out.
Background, then event.
Je lisais quand le téléphone a sonné.
I was reading when the phone rang.
Je suis allé au marché et j'ai acheté du pain.
I went to the market and bought bread.
Two completed events.
Habits and descriptions take the imparfait
Anything that used to happen repeatedly goes in the imparfait, which maps cleanly onto English used to and would. Physical and emotional descriptions of how things were also take it, including age, weather and states of mind.
Quand j'étais jeune, je jouais au foot.
When I was young I used to play football.
Il faisait froid et j'avais peur.
It was cold and I was afraid.
Description, not event.
Some verbs shift meaning
A handful of verbs translate differently in the two tenses, because the passé composé marks the moment a state began rather than the state itself. These come up constantly and the difference is not subtle.
Je savais / J'ai su
I knew / I found out
Je connaissais / J'ai connu
I knew (someone) / I met
Je voulais / J'ai voulu
I wanted / I decided to, I tried
Je pouvais / J'ai pu
I was able to / I managed to
Common mistakes
Hier, je lisais pendant deux heures.
Hier, j'ai lu pendant deux heures.
A stated, finished duration is a completed event, so the passé composé — even though reading feels ongoing.
Quand j'étais petit, je suis allé à la mer chaque été.
Quand j'étais petit, j'allais à la mer chaque été.
Chaque été marks a habit, which takes the imparfait.
Je connaissais ma femme en 2010.
J'ai connu ma femme en 2010.
The passé composé of connaître means met. The imparfait would say you already knew her throughout 2010.
Frequently asked questions
Is there a quick test for choosing?
Ask whether you are saying what happened or what things were like. What happened takes the passé composé; what things were like takes the imparfait. Where it is still unclear, check for a boundary — a stated duration, a start or an end usually means passé composé.
Can both appear in the same sentence?
Constantly, and that is the normal pattern. Je dormais quand tu as appelé puts the background in the imparfait and the interrupting event in the passé composé. Any sentence with an interruption in it needs both.
What about the passé simple?
It is the written narrative past, used in literature and formal history writing and essentially never in speech. You need to recognise it to read novels, but you never have to produce it.