Good morning! I have to leave early today because I have a doctor's appointment. Is that okay?
Sure. I'm very happy you've just given me the introduction to today's lesson!
Today we will talk about all kinds of obligations and necessities. Let's go back to your phrase. I want you to tell me which part expresses that it is mandatory:
I have to leave early today.
I have to expresses that I'm supposed to leave early today.
Exactement. And in French you would use a form of the verb devoir to express this meaning. Since the conjugation is quite irregular, I'll show you another table:
Using the table you can now fill in the blank with the right form.
Je partir tôt.
I have to leave early.
Je dois partir tôt ?
Parfait. Here are some more examples:
Je dois partir à 15h.
I must / have to leave at 3pm.
Tu dois boire plus d'eau.
You must / have drink more water.
Il doit se coucher tôt.
He must / has to go to bed early.
So, have to and must in English are devoir in French?
Very observant! And yes, that's absolutely right.
OK, but what if I don't want to be so direct but rather give an advice?
Then you can use the conditional form of devoir:
Tu devrais boire plus d'eau.
You should drink more water.
Il devrait se coucher tôt.
He should go to bed early.
There's another form, even simpler than devoir: il faut. The good news is that you don't have to conjugate it! However, you can't address people with this form, it's more like saying "it is necessary".
Il faut boire plus d'eau.
Tudois boire plus d'eau.
Bien sûr ! Je dois faire des exercices maintenant !