The verb devoir (to have to, must)

The verb devoir (must, to have to) is an irregular verb with various meanings.
je dois -  I must
tu dois -  you must
il/elle doit -  he/she must
nous devons -  we must
vous devez -  you must
ils/elles doivent -  they must

Let’s start with the notion of debt in the literal and figurative meanings:

Devoir as a debt:

Combien est-ce qu’on vous doit?
How much do we owe you?
Elle me doit mille euros.
She owes me a thousand euros.
Je vous dois quelque chose?
Do I owe you anything?
Il doit sa vie à son chirurgien.
He owes his life to his surgeon.

Devoir as an obligation:

Vous devez assister à la réunion
You must attend the meeting
Demain matin.
Tomorrow morning.
L’enfant doit obéir à ses parents.
The child must obey his parents.
Vous ne devez pas y aller.
You must not go there.

Devoir as a probability:

L’avion doit atterrir à dix heures.
The plane is supposed to land at ten o’clock.
Son livre doit sortir la semaine prochaine.
Her book is supposed to be published next week.
The voice intonation will often determine if devoir implies an obligation or a probability. For example, il doit venir ce soir, may mean he is supposed to come tonight or he must come tonight. Pay attention to intonation and gestures.

Devoir as a warning and suggestion:

When used in the conditional or the past conditional, devoir takes on the meaning of should or should have.
Tu ne devrais pas lui prêter d’argent.
You should not lend him any money.
Elles devraient apprendre une langue étrangère.
They should learn a foreign language.