Devoir is an irregular verb, and it is unusual because it can mean to owe, or it can mean have to, must, ought to and should. Sometimes it is difficult to decide which tense of devoir to use because must, should and ought to don’t exist in all the tenses in English. If in doubt, think of have to: had to means use past tense, will have to is future tense, and would have to is conditional tense.

Present Tense
Je dois – I owe, I have to, I should, I must
Tu dois – you owe, you have to, you should, you must (sing.fam.)
Il doit – he/it owes, he/it has to, he/it should, he/it must
Elle doit – she/it owes, she/it has to, she/it should, she/it must
Nous devons – we owe, we have to, we should, we must
Vous devez – you owe, you have to, you should, you must (pol.pl.)
Ils doivent – they (m.) owe, they have to, they should, they must
Elles doivent – they (f.) owe, they have to, they should, they must

(sing.fam) – singular familiar
(pol.pl) – polite plural

Future tense
Je devrai finir mon travail avant de partir. – I will have to finish my work before leaving.
Tu devras lui parler demain. – You should (will have to) talk to him tomorrow.
Il nous devra beaucoup d’argent. – He will owe us a lot of money.
Nous devrons retourner en France un jour. – We will have to go back to France one day.
Vous devrez leur remercier la prochaine fois. – You should (will have to) thank them next time.
Elles devront rester chez nous. – They will have to stay at our house.

Perfect tense/passé composé
J’ai dû lui preter de l’argent. – I had to lend him some money.
Tu as dû oublier ton sac dans l’autobus. – You must have left (forgotten) your bag on the bus.
Elle a dû leur dire la vérité. – She had to tell them the truth.
Nous avons dû quitter tôt. – We had to leave early.
Vous avez dû vous tromper. – You must have made a mistake.
Ils ont dû passer la nuit dans un hotel. – They had to spend the night in a hotel.

Present subjunctive
Je regrette que je vous doive toujours de l’argent. – I’m sorry that I still owe you some money.
J’espère que tu ne doives pas refaire le travail. – I hope you don’t have to do the work again.
Il est content, bien qu’il doive partir. – He is happy, although he has to leave.
Il semble que nous devions attendre. – It seems that we have to wait.
C’est la chose la plus difficile que vous deviez faire. – It’s the hardest thing you have to do.
Il est possible qu’elles doivent vendre la maison. – They may have to sell the house.

Imperative
The imperative of devoir is regular but is very rarely used, because of its meaning:

dois – owe
devons – let’s owe
devez – owe

With thanks to Elizabeth Allen

Read more on irregular verbs and on learning French

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