Learn Must and Mustn't: Expressing Rules and Importance (A1 English Grammar) — A1 English grammar lesson by Langcom

Learn Must and Mustn’t: Expressing Rules and Importance (A1 English Grammar)

Raúl Maguiña

Fondateur de Langcom | Examinateur pour les examens de Cambridge et du SIELE

septembre 24, 2026

A1✏️ Grammaire⏱️ 3 min de lecture📅 Mise à jour en septembre 2026
Nous utilisons must to talk about things that are very important or necessary to do. It’s like saying ‘you have to do this’ or ‘it is a rule’. For example, You must listen to your teacher. This means it is a very important rule, and you have no choice. We also use must for strong advice or when we are sure about something. For things that are not allowed or are forbidden, we use mustn’t (which is short for must not). It means ‘you cannot do this’ or ‘it is against the rules’. For instance, You mustn’t touch the fire. This is a rule that says touching fire is dangerous and not allowed. Remember, must et mustn’t are modal verbs. They do not change form for different subjects (like I must, he must, they must), and we always use the base form of the main verb after them.

What is ‘must’ for?

Nous utilisons must to talk about things that are very important. It shows a strong obligation or a rule.

  • Obligation or Necessity: When something is necessary to do. For example, Students must wear a uniform.
  • Strong Advice: When you think something is a very good idea. For example, You must visit the museum; it’s amazing!

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What is 'mustn't' for?

Nous utilisons mustn't (ou must not) to talk about things that are not allowed. It shows a prohibition.

  • Prohibition: When something is forbidden or against the rules. For example, You mustn't take photos in the museum.
  • Danger: When something is dangerous to do. For example, Children mustn't play with knives.

How do we form sentences with 'must' and 'mustn't'?

Must et mustn't are easy to use! They are modal verbs, so they follow simple rules.

  • Always use the base form of the verb: Après must ou mustn't, the next verb is always in its basic form (e.g., go, eat, study).
  • No 'to': We do not use to after must ou mustn't.
  • No '-s' for 'he/she/it': Must never changes its form, even for he, she, ou it.
Forms of <span data-no-translation>Must</span> et <span data-no-translation>Mustn't</span>
Objet Positive Form Negative Form
I / You / We / They must go mustn't go
He / She / It must go mustn't go

Exemples

  • I must finish my homework before dinner.
  • You must be quiet in the library.
  • He must wear a helmet when he rides his bike.
  • We mustn't talk during the exam.
  • She mustn't forget her passport for the trip.
  • They must arrive on time for the meeting.

Entraînement

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Erreurs courantes chez les hispanophones

Many Spanish speakers make small mistakes with must et mustn't. Here are the most common ones to avoid:

  • Utilisation de ' to ' après must ou mustn't:
    • You must to study.
    • You must study.
  • Adding '-s' to must for 'he', 'she', 'it':
    • He musts go.
    • He must go.

Questions fréquemment posées

What is the main use of 'must'?

Must is for strong rules, obligations, or things that are very important to do. For example, You must listen to your parents.

What is the main use of 'mustn't'?

Mustn't is for things that are not allowed or forbidden. For example, You mustn't run in the corridors.

Can I say 'must not' instead of 'mustn't'?

Oui, must not is the full form, and mustn't is the short, common form. They mean the same thing.

Does 'must' change for 'he/she/it'?

Non, must never changes its form. It is always must for all subjects (I, you, he, she, it, we, they).

Leçons associées

À propos de l'auteur : Raúl Maguiña

Raúl Maguiña est le directeur de l'Asociación Educativa Langcom, un centre agréé de préparation à l'examen Cambridge English situé à votre pays. Fort de plus d'une décennie d'expérience dans l'enseignement des langues et les certifications internationales, il a aidé des milliers d'étudiants, d'enseignants et d'établissements d'enseignement dans 25 pays à obtenir les certifications d'anglais Cambridge, IELTS, TOEFL et Michigan.

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