Lesson

Must, mustn't, don't need to

Obligation and necessity

≈ 15 min 8 block(s)

Text

Must, mustn't, don't need to

This lesson explains how to use must (obligation), mustn't (prohibition) and don't need to (no obligation). You will learn when to require an action, when to forbid it, and how to say an action is not necessary.

  • Use must + base verb to express a strong obligation or rule (present/future meaning).
  • Use mustn't + base verb to express that something is forbidden.
  • Use don't/doesn't need to + base verb to say an action is not necessary (no obligation).

Table

Summary: must / mustn't / don't need to

Expression Meaning Example
must strong obligation, requirement You must submit the report by Friday.
mustn't strong prohibition, forbidden You mustn't share confidential data with external parties.
don't/doesn't need to no obligation, not necessary You don't need to attend the meeting if you've already reviewed the minutes.
have to (compare) external obligation or necessity I have to finish this by today because the client demands it.

Tip

Key rule: Forms and placement

Remember the basic forms:

  • must + base verb: You must sign the contract.
  • mustn't + base verb: Staff mustn't disclose passwords.
  • do/doesn't need to + base verb: You don't need to send a copy.

For past obligations, use had to (not must). In questions, use modal inversion: Must I...? (less common) or Do I need to...?

Example

Examples in context

You must submit the financial report by Monday.

Employees mustn't share their login details.

You don't need to attend the training if you already completed it.

Managers must approve expense claims before payment.

Tip

Common mistakes to avoid

Watch out for these universal errors:

  • Using 'must' for past obligations (wrong: He musted; correct: He had to).
  • Adding 'to' after must (wrong: must to submit).
  • Confusing must (internal obligation) with have to (external obligation or rules).
  • Incorrect subject-verb agreement after don't need to (wrong: She don't need to).

Always follow a modal with the base verb; for negative necessity use don't/doesn't need to + base verb.

Quiz

Choose the correct sentence:

Hint: Check modal + base verb form.

Quiz

Complete: Employees _____ share confidential passwords.

Hint: Think about expressing a strong prohibition.

Quiz

Which sentence expresses no obligation?

Hint: Look for the phrase that removes necessity rather than giving commands or prohibitions.

Key Points

GrammarPoint

must

modal verb CEFR A2 //mʌst//

used to express a strong obligation or necessity

You must follow company policies.

GrammarPoint

mustn't

modal verb CEFR A2 //ˈmʌsənt//

used to say that something is forbidden

You mustn't share client data without permission.

GrammarPoint

don't need to

expression CEFR B1 //doʊnt niːd tuː//

used to say that something is not necessary; there is no obligation

You don't need to print the document; it's online.

GrammarPoint

have to

modal expression CEFR A2 //hæv tuː//

expresses obligation often from external sources (rules, others)

I have to attend the client meeting this afternoon.

Vocabulary

obligation

noun CEFR B2 //ˌɒblɪˈɡeɪʃən//

a duty or something you must do

Meeting deadlines is a professional obligation.

Vocabulary

prohibition

noun CEFR B2 //ˌprəʊɪˈbɪʃən//

an official rule that stops something from being done

The new policy imposes a prohibition on personal use of company cars.