When to use "devoir" in L'Imparfait or Le Passé Composé

The verb devoir usually means must / to have to (see Conjugate devoir in Le Présent (present tense)).

However, in past context, its meaning changes depending on whether you use Passé composé or Imparfait.

Devoir in Imparfait was supposed to [do]

Il devait me rappeler, mais il a oublié !He was supposed to call me back, but he forgot!

Ils devaient venir ce soir, mais ils ont annulé à la dernière minute.They were supposed to come tonight, but they cancelled at the last minute.

Nous devions nous rejoindre à 17h. Qu'est-ce qui s'est passé ?We were supposed to meet at 5pm. What happened ?

Maintenant que ses parents n'étaient plus là, elle devait s'occuper de sa sœur.Now that her parents were gone, she had to take care of her sister.
-> Here we don't know if she fulfilled that obligation.

When conjugated in Imparfaitdevoir refers to a past obligation, without specifying whether it was met or not.
Actually, in most cases, the obligation was not met.

To conjugate devoir in Imparfait, see Conjugate regular verbs in L'Imparfait (imperfect tense)

Devoir in Passé composéhad to [do] /  must have [done]

Il est parti tôt, donc j'ai dû faire la lessive moi-même.He left early, so I had to do the laundry myself.

Nous étions en retard, donc nous avons dû appeler un taxi.We were late, so we had to call a taxi.

Il a dû le laisser là pour toi.He must have left it there for you.

J'ai trouvé l'écharpe de Sophie. Elle a dû l'oublier.I found Sophie's scarf. She must have forgotten it.

When conjugated in Passé composédevoir can either refer to:
- a past obligation that was met.
or
- the likeliness of an action having taken place, a hypothesis on a past situation.

To conjugate devoir in Passé composé, see Conjugate voir, devoir, pouvoir, boire, croire, savoir, lire, taire (+ avoir) in Le Passé Composé (conversational past)

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Examples and resources

Nous devions nous rejoindre à 17h. Qu'est-ce qui s'est passé ?We were supposed to meet at 5pm. What happened ?
Il a dû le laisser là pour toi.He must have left it there for you.
Nous étions en retard, donc nous avons dû appeler un taxi.We were late, so we had to call a taxi.
Il devait me rappeler, mais il a oublié !He was supposed to call me back, but he forgot!
J'ai trouvé l'écharpe de Sophie. Elle a dû l'oublier.I found Sophie's scarf. She must have forgotten it.
Ils devaient venir ce soir, mais ils ont annulé à la dernière minute.They were supposed to come tonight, but they cancelled at the last minute.
Maintenant que ses parents n'étaient plus là, elle devait s'occuper de sa sœur.Now that her parents were gone, she had to take care of her sister.
Il est parti tôt, donc j'ai dû faire la lessive moi-même.He left early, so I had to do the laundry myself.
Thinking...