Expressing Prohibition: Must Not, Cannot, and May Not

To express that an action is strictly forbidden, illegal, or not allowed by rules and authority, we use must not (mustn’t), cannot (can’t), and may not.

1. Must Not / Mustn’t (Strict / Direct Prohibition)

Must not is the strongest way to express prohibition. It indicates that doing the action is extremely dangerous, illegal, or completely against the rules.

  • Structure: Subject + must not (mustn't) + Verb 1
  • Use: Public signs, laws, safety warnings, or strict parental instructions.
  • Example: You must not smoke in this building. (It is strictly forbidden).
  • Example: We must not forget our passports. (It would cause serious trouble).

2. Cannot / Can’t (No Permission / Impossible under Rules)

Cannot or can’t is a very common, natural way in spoken English to say that something is not allowed or impossible due to rules.

  • Structure: Subject + cannot (can't) + Verb 1
  • Use: Everyday rules, social restrictions, or general lack of permission.
  • Example: You cannot park here; it is a loading zone. (It is against the parking rules).
  • Example: Children can’t watch this movie. It is rated R.

3. May Not (Formal / Written Rules)

May not is a highly formal way to express prohibition. It is typically found in written rules, exams, official documents, and formal speech.

  • Structure: Subject + may not + Verb 1
  • Use: Official settings, formal announcements, guidelines.
  • Example: Candidates may not use calculators during the exam. (Exam regulations).
  • Example: Visitors may not enter the laboratory without permission.

4. Key Differences Summary

ModalFormalityTone / NuanceExample
Must notStrongStrict, critical warningYou must not touch that wire.
Can’tNeutralCasual rule / Social constraintWe can’t wear jeans at work.
May notFormalOfficial guidelines / Written lawsStudents may not leave before noon.

Next, we will explore Lack of Necessity to learn how to express when an action is optional and not required.