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

Fundador de Langcom | Examinador de Cambridge y del SIELE

septiembre 24, 2026

A1✏️ Gramática⏱️ 3 minutos de lectura📅 Actualizado en septiembre de 2026
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Utilizamos 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 y 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?

Utilizamos 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?

Utilizamos mustn't (o 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 y mustn't are easy to use! They are modal verbs, so they follow simple rules.

  • Always use the base form of the verb: After must o mustn't, the next verb is always in its basic form (e.g., go, eat, study).
  • No 'to': We do not use to después de must o mustn't.
  • No '-s' for 'he/she/it': Must never changes its form, even for he, she, o it.
Forms of <span data-no-translation>Must</span> y <span data-no-translation>Mustn't</span>
Asunto Forma positiva Forma negativa
I / You / We / They must go mustn't go
He / She / It must go mustn't go

Ejemplos

  • 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.

Práctica

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Errores habituales de los hispanohablantes

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

  • El uso de 'to' después de must o mustn't:
    • You must to study.
    • You must study.
  • Adding '-s' to must for 'he', 'she', 'it':
    • He musts go.
    • He must go.

Preguntas más frecuentes

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'?

Sí, 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'?

No, must never changes its form. It is always must para todos los sujetos (yo, tú, él, ella, ello, nosotros, ellos).

Lecciones relacionadas

Sobre el autor: Raúl Maguiña

Raúl Maguiña es el director de la Asociación Educativa Langcom, un centro autorizado de preparación para el examen Cambridge English en tu país. Con más de una década de experiencia en la enseñanza de idiomas y la certificación internacional, ha ayudado a miles de estudiantes, profesores e instituciones educativas de 25 países a conseguir las certificaciones de inglés de Cambridge, IELTS, TOEFL y Michigan.

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