Il faut, generally. Devoir states an obligation the speaker acknowledges; il faut presents it as simply necessary, with no room to negotiate. That is why instructions and warnings reach for il faut and advice reaches for tu devrais.

How do I say “you don’t have to”?

Not with il ne faut pas, which means you must not, the same trap English has with mustn’t. For “it isn’t necessary”, use tu n’es pas obligé de or ce n’est pas la peine. Getting this wrong reverses a permission into a prohibition.

Is il faut que always followed by the subjunctive?

Always, without exception. If you find an il faut que followed by an indicative, it is an error rather than a variant. That reliability makes it the single best sentence frame for practising the subjunctive.

Which one to useB1

Devoir vs falloir

The short answer

Devoir conjugates and names who is obliged: je dois partir. Falloir is impersonal and exists only as il faut, so it states a necessity without naming anyone. Il faut plus an infinitive is general; il faut que plus a subjunctive names the person.

At a glance

Devoir or Falloir?

Compare	DevoirConjugates. Names who must.	FalloirImpersonal. States a necessity.
Subject	Any: je, tu, elle, nous	Only il, and it means nothing
Strength	Obligation, often personal	Necessity, generally stronger
With an infinitive	je dois partir	il faut partir: general, no person
With que	Not used	il faut que + subjunctive
Other meanings	To owe, probability: il doit être tard	Needing a thing: il me faut du temps
Register	Neutral	Very common in speech

How to choose

Ask these, in order

1
Do you want to name who is obliged, in a simple sentence?
Devoir, conjugated for that person.
Nous devons partir avant six heures.
We have to leave before six.
2
Is it a general necessity, true for anyone?
Il faut plus an infinitive. No person is named and none is implied.
Il faut réserver à l’avance.
You have to book in advance.
NB“You” here is people in general, which is exactly what il faut expresses.
3
Do you want falloir but with a specific person?
Il faut que plus the subjunctive.
Il faut que tu partes maintenant.
You need to leave now.
4
Are you guessing rather than obliging?
Devoir. It doubles as the verb of probability.
Il doit être malade, il n’est jamais absent.
He must be ill, he’s never off.
NBFalloir cannot express probability at all.

Side by side

Each one, in full

Devoir

When to use devoir

Conjugates. Names who must.

Devoir is an ordinary verb with a full conjugation, so it names who is under the obligation: je dois, tu dois, elle doit. It is the natural choice whenever the sentence already has a subject you want to keep.

It has two other lives worth knowing. It means to owe: je te dois vingt euros, and it expresses probability, which in practice is one of its commonest uses: ça doit coûter cher, that must be expensive.

Tu devrais te reposer.

You should rest.

NBThe conditionnel softens it into advice.

J’ai dû annuler la réunion.

I had to cancel the meeting.

Elle doit avoir une trentaine d’années.

She must be about thirty.

Falloir

When to use falloir

Impersonal. States a necessity.

Falloir exists only in the third person: il faut, il fallait, il faudra, il a fallu. The il refers to nothing at all: it is a grammatical placeholder, like the it in English “it is necessary”.

With an infinitive it states a general rule. With que it narrows to a person and pulls the subjunctive: il faut que je parte, il faut qu’elle vienne. It can also take an indirect object for needing a thing: il me faut deux jours.

Il faut faire attention ici.

You have to be careful here.

Il faudra qu’on en reparle.

We’ll need to talk about it again.

Il me faut encore une heure.

I need another hour.

NBThe person arrives as an indirect object rather than as the subject.

Mistakes to avoid

What learners actually get wrong

Je faut partir.
Il faut que je parte. / Je dois partir.
Falloir has no first person and never will. It exists only as il faut. Both fixes are correct French; the choice between them is about how strong you want the sentence to sound.
Il faut que je pars.
Il faut que je parte.
Il faut que is one of the strongest subjunctive triggers in the language. It is also one of the first that learners meet, which is why it is worth learning as a fixed pattern rather than reasoned about.
Il doit que nous partions.
Il faut que nous partions.
Devoir never takes que. If you want a que clause, the verb has to be falloir, devoir attaches directly to an infinitive instead.
Il faut être malade, il n’est jamais absent.
Il doit être malade, il n’est jamais absent.
Probability is devoir’s job. Il faut être malade would mean one is required to be ill, which is a rule rather than a guess.

Questions

Asked and answered

Which is stronger?: Il faut, generally. Devoir states an obligation the speaker acknowledges; il faut presents it as simply necessary, with no room to negotiate. That is why instructions and warnings reach for il faut and advice reaches for tu devrais.
How do I say “you don’t have to”?: Not with il ne faut pas, which means you must not, the same trap English has with mustn’t. For “it isn’t necessary”, use tu n’es pas obligé de or ce n’est pas la peine. Getting this wrong reverses a permission into a prohibition.
Is il faut que always followed by the subjunctive?: Always, without exception. If you find an il faut que followed by an indicative, it is an error rather than a variant. That reliability makes it the single best sentence frame for practising the subjunctive.

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