Devoir, avoir besoin de and il faut all point at some kind of need or duty in French. They are not the same, and mixing them up can change the sense of a sentence. This article lays out what each one does, when to pick it, and what else you can say instead.

Devoir

Devoir is a verb. It changes form with the subject, like any other French verb. It covers three main jobs.

First, it states a duty or a rule laid on someone from outside, close to the English “must” or “have to.”

• Je dois finir ce travail avant lundi. “I must finish this work before Monday.”
• Tu dois appeler ta mère. “You have to call your mother.”
• Il doit prendre le train de huit heures. “He has to catch the eight o’clock train.”
• Elle doit nettoyer la maison avant midi. “She has to clean the house before noon.”
• Nous devons partir tôt demain. “We must leave early tomorrow.”
• Vous devez signer ici. “You need to sign here.”
• Ils doivent attendre leur tour. “They have to wait their turn.”
• On doit respecter les règles. “You have to follow the rules.”

Second, it gives advice, close to “should” or “ought to.”

• Tu devrais te reposer un peu. “You should rest a little.”
• Il devrait parler à son patron. “He should talk to his boss.”
• Nous devrions arriver plus tôt la prochaine fois. “We should arrive earlier next time.”

Note the form change here. Devrais, devrait, devrions are the conditional forms, and they carry the softer sense of “should” rather than the harder sense of “must.”

Third, devoir can state a guess based on what you know, close to “must be” in English.

• Il doit être fatigué après ce voyage. “He must be tired after that trip.”
• Elle doit avoir trente ans. “She must be thirty.”
• Ils doivent déjà être arrivés. “They must have arrived already.”

Avoir besoin de

Avoir besoin de means “to need.” Unlike devoir, it does not point at a rule set by someone else. It points at a lack or a want felt by the subject. It can be followed by a noun or by a verb in the infinitive form.

• J’ai besoin de dormir. “I need to sleep.”
• Tu as besoin d’argent. “You need money.”
• Il a besoin de repos. “He needs rest.”
• Elle a besoin de rentrer chez elle. “She needs to go home.”
• Nous avons besoin de ton aide. “We need your help.”
• Vous avez besoin d’un visa pour ce pays. “You need a visa for that country.”
• Ils ont besoin de plus de temps. “They need more time.”
• On a besoin de changer de stratégie. “We need to change strategy.”

The line between devoir and avoir besoin de is worth marking clearly. Je dois partir means “I have to leave,” most likely because of a rule, a plan, or a fact outside you, such as a train timetable. J’ai besoin de partir means “I need to leave,” and points at your own state, such as tiredness or stress, rather than an outside rule.

Il faut

Il faut never changes form. It stays il faut no matter who is involved. It states a need or a duty in a general way, without naming who must act, though the person is often clear from context.

Il faut plus an infinitive states a general duty that could apply to anyone.

• Il faut réserver à l’avance. “You have to book in advance.”
• Il faut être patient. “One must be patient.”
• Il faut y aller. “We have to go.”
• Il faut travailler dur pour réussir. “You have to work hard to succeed.”

Il faut plus a noun states that something is needed, often in the sense of a fixed amount or a fixed thing.

• Il faut du temps pour apprendre une langue. “It takes time to learn a language.”
• Il faut deux heures pour arriver là-bas. “It takes two hours to get there.”
• Il faut de la patience pour ce métier. “This trade takes patience.”

Il faut is common and sits well in everyday talk. It sounds less pointed than naming a person with devoir, since it frames the duty as a fact of the situation rather than an order aimed at someone.

Il faut que

When you want to name exactly who must act, il faut pairs with que and a verb in the subjunctive mood. This form is used often in spoken French.

• Il faut que tu partes maintenant. “You have to leave now.”
• Il faut que je finisse ce rapport ce soir. “I have to finish this report tonight.”
• Il faut qu’il vienne demain. “He has to come tomorrow.”
• Il faut qu’elle appelle le médecin. “She has to call the doctor.”
• Il faut que nous trouvions une solution. “We need to find a solution.”
• Il faut que vous compreniez la situation. “You need to understand the situation.”
• Il faut qu’ils comprennent les règles. “They need to understand the rules.”
• Il faut qu’on parle. “We need to talk.”

The subjunctive forms of common verbs used after il faut que are worth knowing by heart, since they often differ from the plain present tense. Que je sois, que tu aies, qu’il fasse, que nous allions, and so on.

How to choose

• Name a duty tied to a specific person, driven by a rule, plan, or fact outside them: use devoir.
• Give advice: use devoir in the conditional form, devrais, devrait, devrions, and so on.
• State a guess based on the facts at hand: use devoir.
• Point at a personal need or lack, not an outside rule: use avoir besoin de.
• State a general duty or need that applies broadly, without naming a person: use il faut plus the infinitive or plus a noun.
• Name the exact person who must act, using a full clause: use il faut que plus the subjunctive.

Synonyms and alternative expressions

Être obligé de plus an infinitive states an obligation, close in force to devoir.

• Je suis obligé de partir tôt. “I am obliged to leave early.”
• Elle est obligée de refaire son travail. “She is obliged to redo her work.”

Avoir à plus an infinitive is a plainer, somewhat formal way to state a task ahead of you.

• J’ai à faire quelques courses. “I have some errands to run.”
• Nous avons à revoir ce projet. “We have to look at this project again.”

Il est nécessaire de plus an infinitive is a formal way to state that something is needed. It reads better in writing than in speech.

• Il est nécessaire de vérifier les faits. “It is necessary to check the facts.”

Il est indispensable de plus an infinitive states a strong need, close to “essential.”

• Il est indispensable de réserver une table. “It is essential to book a table.”

Devoir absolument adds force to a duty already stated with devoir.

• Je dois absolument te parler. “I really must talk to you.”

Manquer de means “to lack,” and points at a missing thing rather than a duty.

• Cette plante manque d’eau. “This plant lacks water.”
• Il manque de confiance en lui. “He lacks confidence.”

Exiger means “to require” or “to demand.” It is formal and takes a direct object, unlike devoir and il faut.

• Ce poste exige de l’expérience. “This job requires experience.”
• La loi exige que chaque conducteur ait une assurance. “The law requires every driver to have insurance.”

Summary

• Devoir names a duty tied to a person, gives advice in the conditional form, or states a guess.
• Avoir besoin de points at a personal need or lack, not a rule set by someone else.
• Il faut states a general need or duty, without naming a person, and pairs with an infinitive or a noun.
• Il faut que names the exact person who must act, and pairs with the subjunctive.
• Être obligé de, avoir à, il est nécessaire de, and il est indispensable de offer other ways to state duty or need, each with its own weight and register.