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English grammar

Must and Mustn't (Strong Obligation)

Updated 27 Oct 2026 4 min read LingoLadder Editorial Team
Cover: must and mustn't — a to-do list for strong obligation and a no-smoking sign for prohibition
Short answer

Use must for a strong obligation you feel is necessary: 'I must finish this today'. Use mustn't to say something is forbidden: 'You mustn't smoke here'. Both are followed by the base verb with no 'to'.

On this page10 sections · tap to jump
  1. 01The rule
  2. 02Must, mustn’t and have to — the forms
  3. 03Example sentences
  4. 04Common mistakes
  5. 05A short conversation
  6. 06Usage notes
  7. 07Compared with…
  8. 08Practice
  9. 09Quiz
  10. 10Recap cheat-sheet

Level: B1. Must and mustn’t are modal verbs — helping verbs that add meaning to the main verb. Must says something is strongly necessary; mustn’t says something is forbidden. The trap is that these two are not opposites, and this lesson shows you exactly where learners slip.

The rule

A modal verb goes before the base form of another verb and never changes its own ending: I must go, she must go (never musts). After must and mustn’t you use the base verb with no to: must go, not must to go.

Must expresses a strong obligation — something you feel is necessary or very important. Often the pressure comes from the speaker: I must call the doctor means I myself feel it is necessary. Mustn’t (short for must not) expresses prohibition — it is forbidden, do not do it: You mustn’t touch that wire.

Here is the part that causes most errors. The real opposite of must is don’t have to, which means there is no obligation — you can if you want, but you don’t need to. Mustn’t is something else entirely: it means the action is not allowed. Compare You don’t have to wear a tie (optional) with You mustn’t wear jeans (forbidden).

Must has no past or future form of its own. For the past you borrow had to; for the future you use will have to: Yesterday I had to work late, Tomorrow you will have to leave early.

Must, mustn’t and have to — the forms

Meaning Form Example
strong obligation (present) must + base verb You must wear a helmet.
prohibition (present) mustn’t + base verb You mustn’t park here.
no obligation (present) don’t / doesn’t have to You don’t have to come.
obligation (past) had to + base verb I had to wait an hour.
obligation (future) will have to + base verb She will have to decide soon.

Example sentences

  1. I must finish this report today.
    Strong obligation the speaker feels.
  2. You must be quiet in the library.
    A rule stated as a strong obligation.
  3. We must leave now or we'll miss the train.
    Necessity because of the situation.
  4. She must see a doctor about that cough.
    The speaker thinks it is really necessary.
  5. You mustn't smoke in here.
    Prohibition — it is forbidden.
  6. Children mustn't play near the road.
    Forbidden for safety.
  7. You mustn't tell anyone about this.
    A strong instruction not to do it.
  8. Passengers mustn't lean out of the window.
    A rule that forbids an action.
  9. I must remember to lock the door.
    A note-to-self obligation.
  10. We mustn't be late for the interview.
    It is very important not to.
  11. You must try this cake, it's delicious.
    A strong recommendation, also with must.
  12. Drivers must stop at a red light.
    A legal obligation.
  13. You mustn't use your phone during the exam.
    Forbidden by the exam rules.
  14. I had to work late last night.
    Past obligation — must has no past, use had to.
  15. You will have to book a ticket in advance.
    Future obligation — use will have to.
  16. You don't have to pay now; next week is fine.
    No obligation — not the same as mustn't.
  17. He must apologise for what he said.
    The speaker judges it necessary.
  18. We mustn't forget her birthday again.
    A strong warning to ourselves.
  19. Visitors must sign in at reception.
    An institutional obligation.
  20. You mustn't drink and drive.
    Forbidden — a serious prohibition.

Common mistakes

  • ❌ You mustn’t come if you are busy. → ✅ You don’t have to come if you are busy. Mustn’t = forbidden; “no need” is don’t have to.
  • ❌ I must to go now. → ✅ I must go now. No to after must.
  • ❌ Yesterday I must work late. → ✅ Yesterday I had to work late. Must has no past; use had to.
  • ❌ She musts wear a uniform. → ✅ She must wear a uniform. Modals never add -s.
  • ❌ You don’t must park here. → ✅ You mustn’t park here. The negative of must for prohibition is mustn’t, not don’t must.
  • ❌ Must I to answer? → ✅ Must I answer? Questions invert the modal and use the base verb, no to.

A short conversation

A: The sign says we mustn’t use flash photography inside. B: Okay. Must we leave our bags at the entrance too? A: No, you don’t have to, but you must keep them with you at all times. B: Got it. Then we must be careful near the paintings.

Usage notes

  • must vs have to. Must often carries the speaker’s own sense of necessity (I must stop eating sugar), while have to points to an outside rule or circumstance (I have to wear a badge at work). In everyday speech, especially in American English, have to is more common for obligations; must can sound formal or official, which is why you see it on signs and in rules.
  • Questions. Must I…? is possible but formal; most speakers ask Do I have to…? for present obligation.
  • No past, no future. Remember the gap: past = had to, future = will have to. Mustn’t likewise has no past; the past of a prohibition is usually wasn’t allowed to or couldn’t.
  • Deduction. Must has a second life as a confident guess: The lights are off — they must be out. That meaning is covered in the deduction lesson below; here we only mean obligation.

Compared with…

  • Have to carries obligation too, but from an external source, and it conjugates fully (has to, had to, will have to, don’t have to). This is why must leans on it for past and future. See Have to and Has to (Obligation).
  • Must for deduction uses the same word for a near-certain conclusion: You’ve been travelling all day — you must be exhausted. The opposite there is can’t, not mustn’t. See Must, Can’t, Might for Deduction.

Recap card for must and mustn’t showing obligation, prohibition and no obligation with example sentences

Quick recap before you practise.

Practice

Practice

Must and mustn't practice

Try each one, then press Check. No sign-up, nothing is sent anywhere — it all runs in your browser.

1 Choose the best option: 'This is a quiet zone. You ___ make noise.'
Show answer

mustn't — Making noise is forbidden here, so use mustn't.

2 Choose the best option: 'The meeting was cancelled, so you ___ come tomorrow.'
Show answer

don't have to — There is no obligation now, so use don't have to — not mustn't (which would forbid it).

Hint: base verb after must

Show answer

go — After must you use the base verb with no 'to': go.

Hint: must has no past form

Show answer

had to — Must has no past; the past obligation is had to.

5 Put the words in order to make a prohibition.

Show answer

You mustn't use your phone — mustn't + base verb expresses prohibition.

6 Match each form to its meaning.

  • must
  • mustn't
  • don't have to
  • had to
Show answer
  • must → strong obligation
  • mustn't → it is forbidden
  • don't have to → no obligation
  • had to → past obligation

Mustn't is prohibition, not the opposite of must; the opposite is don't have to.

Quiz

Check yourself

Must and mustn't quiz

12 questions · pick an answer and you'll see straight away if it's right.

0/12 0 answered
  1. 1Choose: 'You ___ touch that — it's extremely hot.'
    Show answer

    mustn't — Touching it is forbidden for safety, so mustn't.

  2. 2Choose: 'It's a holiday, so we ___ go to work.'
    Show answer

    don't have to — There is no obligation to work, so don't have to. Mustn't would mean it is forbidden.

  3. 3Which sentence is correct?
    Show answer

    I must leave now. — After must, use the base verb with no 'to' and no -s.

  4. 4What is the past of 'I must work late'?
    Show answer

    I had to work late. — Must has no past form; use had to.

  5. 5Choose: 'Visitors ___ sign in at reception.' (it is required)
    Show answer

    must — Signing in is required, so must expresses the obligation.

  6. 6What does 'You mustn't park here' mean?
    Show answer

    Parking here is forbidden. — Mustn't expresses prohibition: it is not allowed.

  7. 7What does 'You don't have to wear a tie' mean?
    Show answer

    Wearing a tie is optional. — Don't have to means there is no obligation; it is optional.

  8. 8Choose the correct question form:
    Show answer

    Must I answer? — Invert the modal and use the base verb: Must I answer?

  9. 9Choose: 'We ___ be late for the flight.' (it is very important not to)
    Show answer

    mustn't — It is important not to be late, so mustn't.

  10. 10Which is the future of 'You must book a ticket'?
    Show answer

    You will have to book a ticket. — Must has no future; use will have to.

  11. Show answer

    mustn't — Using the phone is forbidden; mustn't (must not) expresses prohibition.

  12. 12Which sentence shows no obligation (optional), not prohibition?
    Show answer

    You don't have to shout. — Don't have to means it is optional; the others forbid or require shouting.

Recap cheat-sheet

Meaning Use Example
strong obligation must + base You must stop.
it is forbidden mustn’t + base You mustn’t stop.
no obligation don’t have to You don’t have to stop.
past obligation had to You had to stop.

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125 topics 125 ready now
A1 30 topics · 30 ready
  1. 01 Verb 'To Be' in the Present (am, is, are)
  2. 02 Subject Pronouns: I, You, He, She, It, We, They
  3. 03 Present Simple: Affirmative and the -s Ending
  4. 04 Present Simple: Negatives and Questions (do/does)
  5. 05 Articles: a / an and When to Use Each
  6. 06 The Definite Article 'The': When to Use It
  7. 07 Zero Article: When to Use No Article
  8. 08 Plural Nouns: Regular and Irregular Forms
  9. 09 This, That, These, Those (Demonstratives)
  10. 10 Possessive Adjectives: my, your, his, her
  11. 11 Possessive 's and s' (Showing Belonging)
  12. 12 There Is / There Are (Saying What Exists)
  13. 13 Have Got: Talking About Possession
  14. 14 Can and Can't for Ability
  15. 15 Present Continuous (am/is/are + -ing)
  16. 16 Imperatives: Giving Orders and Instructions
  17. 17 Adjectives: Position and Basic Use
  18. 18 Prepositions of Place: in, on, at
  19. 19 Prepositions of Time: in, on, at
  20. 20 Question Words: What, Where, When, Who, Why, How
  21. 21 How Much vs How Many
  22. 22 Countable and Uncountable Nouns
  23. 23 Some and Any: Which One to Use
  24. 24 Object Pronouns: me, you, him, her, us, them
  25. 25 Adverbs of Frequency and Their Position
  26. 26 Like, Love, Hate + -ing (Expressing Preferences)
  27. 27 Would Like: Polite Wants and Offers
  28. 28 Making Requests: Can, Could, May I
  29. 29 Past Simple of 'To Be' (was / were)
  30. 30 Let's and Shall We: Making Suggestions
A2 25 topics · 25 ready
  1. 31 Past Simple: Regular Verbs and -ed Spelling
  2. 32 Past Simple: Irregular Verbs (go, see, have)
  3. 33 Past Simple: Negatives and Questions (did)
  4. 34 Comparative Adjectives (bigger, more expensive)
  5. 35 Superlative Adjectives (the biggest, the most)
  6. 36 Going To: Plans and Predictions
  7. 37 Will: Predictions, Decisions and Offers
  8. 38 Present Continuous for Future Arrangements
  9. 39 Present Perfect for Experience (ever / never)
  10. 40 Present Perfect with just, already, yet
  11. 41 Present Perfect vs Past Simple
  12. 42 Have To and Don't Have To (Obligation)
  13. 43 Should and Shouldn't for Advice
  14. 44 Could: Past Ability and Polite Use
  15. 45 Much, Many, A Lot Of
  16. 46 A Few, A Little, Few, Little
  17. 47 Adverbs of Manner (quickly, well, hard)
  18. 48 Verb + -ing or Verb + to (Introduction)
  19. 49 Zero Conditional (Facts and Rules)
  20. 50 First Conditional (Real Future Possibility)
  21. 51 Prepositions of Movement (to, into, through)
  22. 52 Possessive Pronouns: mine, yours, hers
  23. 53 Past Continuous (was/were + -ing)
  24. 54 Too and Enough (Degree)
  25. 55 Order of Adjectives Before a Noun
B1 27 topics · 27 ready
  1. 56 Past Continuous vs Past Simple
  2. 57 Used To and Would for Past Habits
  3. 58 Present Perfect Continuous (have been -ing)
  4. 59 Present Perfect: Simple vs Continuous
  5. 60 Past Perfect (had + past participle)
  6. 61 Second Conditional (Unreal Present/Future)
  7. 62 Third Conditional (Unreal Past)
  8. 63 Reported Speech: Statements (Backshift)
  9. 64 Reported Questions
  10. 65 Reported Commands and Requests
  11. 66 Passive Voice: Present and Past Simple
  12. 67 Must and Mustn't (Strong Obligation) You are here
  13. 68 May, Might, Could for Possibility
  14. 69 Defining Relative Clauses (who, which, that)
  15. 70 Non-Defining Relative Clauses (with commas)
  16. 71 Gerunds and Infinitives: Full Verb Patterns
  17. 72 Verbs Where -ing vs to Changes Meaning
  18. 73 Future Continuous (will be -ing)
  19. 74 So and Such (Emphasis)
  20. 75 As...As Comparisons (equality)
  21. 76 Question Tags (isn't it? do you?)
  22. 77 Phrasal Verbs: Separable and Inseparable
  23. 78 Linking Words: because, so, although, but
  24. 79 Infinitive of Purpose (to / in order to)
  25. 80 Indirect Questions (Can you tell me...)
  26. 81 Make, Let, Allow, Help (Causatives Intro)
  27. 82 Be Used To and Get Used To (+ -ing)
B2 25 topics · 25 ready
  1. 83 Narrative Tenses for Storytelling
  2. 84 Past Perfect Continuous (had been -ing)
  3. 85 Mixed Conditionals
  4. 86 Wish and If Only (Regrets and Wishes)
  5. 87 Wish/If Only + would (Complaints)
  6. 88 Passive Voice: All Tenses and Modals
  7. 89 Reporting Passive (It is said that / is said to)
  8. 90 Causative: Have/Get Something Done
  9. 91 Reduced Relative Clauses (Participles)
  10. 92 Modals of Deduction: Present and Past
  11. 93 Needn't Have vs Didn't Need To
  12. 94 Future Perfect (will have done)
  13. 95 Participle Clauses (-ing / -ed / having done)
  14. 96 Advanced Comparatives (far/much + er, the more)
  15. 97 Conditional Alternatives: unless, as long as, provided
  16. 98 Advanced Determiners: each, every, either, neither
  17. 99 Intensifiers and Adverbs of Degree
  18. 100 Advanced Contrast: despite, in spite of, however
  19. 101 Advanced Linkers: therefore, moreover, consequently
  20. 102 Advanced Article Use (generalisations, names)
  21. 103 Verb + Preposition and Adjective + Preposition
  22. 104 Relative Clauses: whose, where, when, quantifier + whom
  23. 105 Subject-Verb Agreement: Tricky Cases
  24. 106 Emphatic Do (I do like it)
  25. 107 Would for Past Habits (Formal Narrative)
C1 18 topics · 18 ready
  1. 108 Inversion with Negative Adverbials
  2. 109 Inversion in Conditionals (Had I known)
  3. 110 Cleft Sentences (It was... / What I need is...)
  4. 111 Fronting and Emphasis (Word Order)
  5. 112 Subjunctive and Mandative (I suggest he be)
  6. 113 Ellipsis and Substitution (so, do, one)
  7. 114 Hedging and Cautious Language
  8. 115 Nominalisation (Academic Noun Style)
  9. 116 Advanced Discourse Markers (Spoken & Written)
  10. 117 Advanced Passive (passive + infinitive, get-passive)
  11. 118 Advanced Comparative Structures (no sooner...than)
  12. 119 Future in the Past (was going to / would)
  13. 120 Advanced Modals (may well, might as well, would rather)
  14. 121 Advanced Reported Speech (Reporting Verbs + Patterns)
  15. 122 Advanced and Idiomatic Phrasal Verbs
  16. 123 Collocation and Colligation (Natural Combinations)
  17. 124 Advanced Concession (much as, whatever, however + adj)
  18. 125 Absolute and Non-Finite Clauses (Formal)