Lesson

Must, mustn't, needn't

Necessity and prohibition

≈ 15 min 8 block(s)

Text

Must, mustn't, needn't

These modal verbs express obligation, prohibition, or lack of necessity. Use them with the base form of the verb (no 'to'). They appear in present-time statements and rules — for past obligation use 'had to'.

  • must = strong obligation or logical necessity (I must finish this report).
  • mustn't = prohibition (You mustn't share passwords).
  • needn't = no obligation; something is not necessary (You needn't come if you already know the update).

Table

Overview: must / mustn't / needn't

Modal Meaning Use Example
must Obligation / necessity Strong requirement from speaker or rules You must submit the report by Friday.
mustn't Prohibition Rules that forbid actions Employees mustn't share confidential passwords.
needn't No obligation Saying something is not necessary You needn't attend if you send your update by email.

Tip

Key Rule: Form and meaning

Remember the form and main differences:

  • Use modal + base verb: must / mustn't / needn't + verb (no 'to').
  • Must expresses obligation or strong necessity; mustn't expresses prohibition.
  • Needn't means something is not necessary (equivalent to 'do not need to').
  • There is no past form of must; use 'had to' for past obligations.

Modal verbs use the base verb after them and do not take -s in the third person.

Example

Examples in context

You must submit the annual budget by Monday.

Employees mustn't disclose client data to external parties.

You needn't attend the training if you watched the recording.

I must finalize the agenda before the board meeting.

Tip

Common mistakes to avoid

Watch out for these universal errors:

  • Confusing mustn't (prohibition) with needn't (no necessity).
  • Adding 'to' after a modal: incorrect -> 'must to', correct -> 'must'.
  • Using 'do/does' to make negatives with modals (e.g., 'do not must') instead of the correct modal negative.
  • Using 'must' for past obligations — use 'had to' instead.

Always use the base verb after must, mustn't, and needn't.

Quiz

Choose the correct sentence:

Hint: Focus on obligation and modal structure.

Quiz

Complete: Employees _____ share confidential client information.

Hint: Think about a workplace rule that forbids an action.

Quiz

Which sentence means 'it is not necessary'?

Hint: Look for the modal that expresses 'not necessary'.

Key Points

GrammarPoint

must

modal verb CEFR B1 //mʌst//

Expresses obligation, necessity, or strong recommendation.

You must finish the contract before signing.

GrammarPoint

mustn't

modal verb CEFR B1 //ˈmʌsnt//

Indicates prohibition; something is not allowed.

Staff mustn't share passwords with external partners.

GrammarPoint

needn't

modal verb CEFR B1 //ˈniːdn̩t//

Shows that something is not necessary; no obligation.

You needn't attend the meeting if you send your report.

Vocabulary

obligation

noun CEFR B2 //ˌɒblɪˈɡeɪʃən//

A duty or requirement to do something.

Meeting the deadline is a contractual obligation.

Vocabulary

prohibition

noun CEFR B2 //ˌprəʊɪˈbɪʃən//

An official rule that forbids something.

There is a prohibition on smoking in all company buildings.

GrammarPoint

base verb (bare infinitive)

grammar term CEFR B1 //beɪs vɜːb/ /ˈbɛə(r) ˌɪnfɪnətɪv//

The verb form without 'to' used after modal verbs (e.g., 'submit', 'attend').

Modals are followed by the base verb: must approve, needn't attend.