1. Obligation and necessityDois-tu étudier ce soir ?
Do you have to study tonight?
Elles doivent manger
They must / need to eat.
2. Probability and suppositionIl doit rentrer avant le dîner
He should / will probably be back before dinner
Nous devons gagner plus cette année
We should earn more this year.
Elle doit être à l'école
She must be at school
3. Expectation and intentionJe devais aller avec eux
I was supposed to go with them.
Il devait le faire, mais il a oublié
He was supposed to do it, but he forgot
4. Fatalism and inevitabilityIl devait perdre un jour
He had to / was bound to lose one day
Elle ne devait pas l'entendre avant lundi
She wasn't to hear it until Monday
Translating devoirDevoir can be translated by should, must, ought to, have to, supposed to - the distinction between necessity and probability is not always clear:
Je dois faire la lessive
I should/must/have to do the laundry
Il doit arriver demain
He is supposed to / should / has to arrive tomorrow
To specify "must" rather than "should," add a word like absolument or vraiment:
Je dois absolument partir
I really have to go
Nous devons vraiment te parler
We must speak to you
To specify "should" rather than "must," use the conditional:
Tu devrais partir
You should leave.
Ils devraient lui parler
They should talk to him.
To say that something that "should have" happened, use the conditional perfect of devoir plus the infinitive:
Tu aurais dû manger
You should have eaten.
J'aurais dû étudier
I should have studied.
Devoir as a transitive verbWhen used transitively (and thus not followed by a verb), devoir means "to owe":
Combien est-ce qu'il te doit ?
How much does he owe you?
Pierre me doit 10 francs
Pierre owes me 10 francs
Conjugations of devoirPresent tense