Must and have to are close in meaning, but their negatives are not. Must not bans an action: "You must not smoke here." Do not have to removes the necessity: "You do not have to come." One closes a door and the other opens it.
The trap is easy to fall into because both forms contain the same idea in the positive. A learner who knows "I must pay" and "I have to pay" may build "I must not pay" and "I do not have to pay" as if they were the same. They are not: one forbids payment, the other says payment is optional.
This page separates the two negatives, shows the past and future forms, and finishes with a 20-question drill. Each answer explains whether the sentence is a ban or a choice.
The rule
Two negatives, two meanings
- Must not and mustn't ban an action: "You mustn't smoke here."
- Do not have to and don't have to remove the necessity: "You don't have to come."
- A ban says no. No necessity says the action is allowed but optional.
- The difference can turn a rule into an invitation, so read carefully.
Mustn't: prohibition
- Use mustn't for rules, signs and strong warnings: "Visitors mustn't touch the paintings."
- Mustn't exists only in the present.
- For the past, use was not allowed to or could not: "We couldn't park there."
- Think of mustn't as one word meaning not allowed.
Don't have to: no obligation
- Use don't have to when something is optional: "You don't have to pay; entry is free."
- The form changes with the subject: don't have to, doesn't have to, didn't have to.
- The past is didn't have to: "I didn't have to work yesterday."
- Will have to covers the future: "You will have to book a table."
Questions and answers
- Questions use do: "Do I have to sign here?"
- A short answer uses don't: "No, you don't."
- Must not is almost never used in questions.
- For permission questions, use can: "Can I leave now?"
Common confusions
- Mustn't is not the negative of have to; don't have to is.
- Needn't means the same as don't have to: "You needn't wait."
- Must for deduction is different again: "He must be tired."
- Read the sentence twice before choosing, because the two negatives sit far apart in meaning.
Examples
- You mustn't use your phone in the exam.A ban: phones are not allowed.
- You don't have to bring your own towel.No necessity: towels are provided, but yours is allowed.
- Visitors mustn't feed the animals.A rule and a prohibition.
- I don't have to work tomorrow.The day is free, but working is not forbidden.
- She doesn't have to wear a uniform.No obligation for the third-person subject.
- We didn't have to pay for parking.The past form of no necessity.
- You will have to book a table for six.The future obligation keeps have to.
- Do I have to sign the form?Questions about obligation use do with have to.
- You needn't hurry; we have time.Needn't means the same as don't have to.
- He must be tired after the journey.Must for deduction is unrelated to prohibition.
Common mistakes
- Wrong: You mustn't pay; the museum is free. Right: You don't have to pay; the museum is free.
- Free entry means payment is unnecessary, not forbidden, so don't have to is correct.
- Wrong: You don't have to touch the wire. Right: You mustn't touch the wire.
- A dangerous action needs a ban, and bans take mustn't.
- Wrong: She doesn't must work tomorrow. Right: She doesn't have to work tomorrow.
- Must has no do-auxiliary form, so the negative of obligation is doesn't have to.
- Wrong: I didn't must hurry. Right: I didn't have to hurry.
- The past of no necessity is didn't have to.
- Wrong: You mustn't to smoke here. Right: You mustn't smoke here.
- Mustn't takes the base verb without to.
Practice drill
Answer all 20 questions. Each answer is marked at once, and the rule is explained under the question.
You ___ use your phone in the exam. It is against the rules.
Show answer and explanation
Answer: A. mustn't A rule that forbids an action takes mustn't.
You ___ bring a towel; the hotel provides one.
Show answer and explanation
Answer: B. don't have to The towel is unnecessary but allowed, so don't have to fits.
Visitors ___ feed the animals. It is forbidden.
Show answer and explanation
Answer: C. mustn't A prohibition takes mustn't, which bans feeding completely.
I ___ work tomorrow; it is a holiday.
Show answer and explanation
Answer: D. don't have to The holiday removes the obligation, so don't have to is correct.
She ___ wear a uniform at her new job.
Show answer and explanation
Answer: A. doesn't have to The third-person negative of no obligation is doesn't have to.
We ___ pay for parking because the first hour was free.
Show answer and explanation
Answer: B. didn't have to The past form of no necessity is didn't have to.
You will ___ book a table for six.
Show answer and explanation
Answer: C. have to The future obligation uses will have to.
___ I have to sign the form?
Show answer and explanation
Answer: D. Do Questions about obligation use the auxiliary do with have to.
You ___ hurry; we have plenty of time.
Show answer and explanation
Answer: A. needn't Needn't means the hurry is unnecessary, the same as don't have to.
Passengers ___ lean out of the window.
Show answer and explanation
Answer: B. mustn't A safety rule that forbids the action takes mustn't.
He ___ get up early on Sundays.
Show answer and explanation
Answer: C. doesn't have to Sunday removes the obligation for he, so doesn't have to is correct.
You ___ pay me back; it was a gift.
Show answer and explanation
Answer: D. don't have to The repayment is unnecessary but not forbidden, so don't have to fits.
The sign says guests ___ use the pool after ten.
Show answer and explanation
Answer: A. mustn't A sign stating a rule uses mustn't for the prohibition.
We ___ leave yet; the train is at nine.
Show answer and explanation
Answer: B. don't have to There is no need to leave yet, so don't have to is correct.
She ___ cook tonight; her friends are bringing dinner.
Show answer and explanation
Answer: C. doesn't have to Bringing dinner removes the need to cook, so doesn't have to fits.
Choose the correct sentence.
Show answer and explanation
Answer: D. You mustn't smoke in the hospital. A ban takes mustn't plus the base verb smoke with no to.
In the past, children ___ wear a uniform at that school.
Show answer and explanation
Answer: A. didn't have to The past of no obligation is didn't have to.
You ___ take a taxi; I can drive you.
Show answer and explanation
Answer: B. don't have to The lift removes the need for a taxi, so don't have to is correct.
"You needn't wait" means ___.
Show answer and explanation
Answer: C. you don't have to wait Needn't and don't have to both express no obligation.
Students ___ use dictionaries during the test. It is not allowed.
Show answer and explanation
Answer: D. mustn't Not being allowed is a prohibition, so mustn't is correct.
Drill complete
You answered every question.
Next topic: Should vs ought to: advice and expectationTake the level test
Questions learners ask
What is the difference between mustn't and don't have to?
Mustn't forbids the action completely: "You mustn't park here" means parking is not allowed. Don't have to says the action is optional: "You don't have to park here" means you may park elsewhere or not at all. One is a rule, the other is a free choice.
Why is mustn't rare in questions?
Mustn't asks whether something is forbidden, and that meaning is usually carried by can or be allowed to: "Can I park here?" or "Are we allowed to leave?" Must not in a question sounds like an old rulebook. Use can for permission and do have to for obligation.
Is needn't the same as don't have to?
Yes. "You needn't wait" and "You don't have to wait" mean the same thing: waiting is not necessary. Needn't is more common in British English, and it has no do-auxiliary form: never "you don't needn't".
More grammar topics
Should vs ought to: advice and expectation
Should and ought to mean almost the same thing. Both give advice and state expectations; should is more common and takes questions and negatives without help.
Past modals of deduction: must have, might have, can't have
Must have shows certainty, might have shows possibility and can't have shows impossibility about the past. All three take have plus the past participle.
Verbs followed by a gerund: enjoy, avoid, suggest
A group of common verbs takes an -ing form after them: enjoy, avoid, suggest, finish, practise and mind. Learn the group as a fixed pattern.
Take the free A1 to C2 level test to find your starting point.