Must and have to both describe obligations, but they come from different places. Must usually comes from the speaker: "You must call your mother." The speaker feels strongly about it. Have to usually comes from outside: "I have to wear a uniform at work." The rule or the situation requires it.
The negatives matter more than the difference above. Must not means something is forbidden. Do not have to means something is not necessary. "You must not smoke here" is a ban; "you do not have to come" means your presence is optional.
This page covers the source of the obligation, the tenses, the two negatives, and must for logical deduction. The drill at the end checks every pattern with a rule explanation.
The rule
Must: the speaker's obligation
- Must expresses an obligation the speaker believes in: "I must finish this today."
- Must suits written rules, notices and instructions: "Passengers must show a ticket."
- Must has no past or future form. Use had to for the past and will have to for the future.
- In questions, must is rare. Use have to: "Do I have to sign here?"
Have to: the obligation from outside
- Have to expresses rules, laws and circumstances: "She has to take medicine every day."
- Have to works in every tense: had to, has had to, will have to, would have to.
- The subject changes the form: I have to, she has to, they have to.
- In negatives, not goes with the auxiliary: do not have to, does not have to, did not have to.
The two negatives
- Must not and mustn't mean it is forbidden: "You must not park here."
- Do not have to means it is not necessary: "You do not have to pay; entry is free."
- The difference is absolute, so choose the negative with care.
- Answering a question, use do not have to for freedom: "Do I have to leave?" — "No, you do not have to."
Must for deduction
- Must can show that something is logically certain: "You must be tired after that journey."
- The opposite deduction uses cannot: "She cannot be at home; her car is gone."
- Must for deduction has no have to equivalent. "You have to be tired" describes an obligation, not a conclusion.
- For past deduction, use must have plus the past participle: "He must have missed the train."
Which one sounds natural?
- In everyday speech, have to is more common for rules and routines: "I have to get up early."
- Must sounds formal and strong, and it appears on signs: "Staff must wash their hands."
- When you give advice, should is often better than must, because must can sound like an order.
- When the obligation is your own decision, either works, and must sounds more personal.
Examples
- I must call my grandmother tonight.A personal obligation the speaker feels strongly about.
- I have to call my bank before noon.An obligation from a deadline and a procedure, outside the speaker.
- Visitors must not touch the paintings.Must not is a prohibition.
- You do not have to bring food; lunch is provided.Do not have to means it is not necessary.
- She had to work last Saturday.Had to is the past form, because must has no past tense.
- We will have to leave before six.Will have to makes the obligation future.
- You must be hungry after the long walk.Must expresses a logical deduction, not an obligation.
- Do I have to book a table?Questions about obligation use have to, not must.
- Children have to wear helmets on the site.A general rule from outside the speaker.
- He must have forgotten the appointment.Must have plus the past participle is past deduction.
Common mistakes
- Wrong: I must to go now. Right: I must go now.
- Must takes the base verb without to. Have to keeps the to: I have to go.
- Wrong: She does not must work tomorrow. Right: She does not have to work tomorrow.
- Must has no do-auxiliary form. The negative of obligation is does not have to.
- Wrong: You must not pay; entry is free. Right: You do not have to pay; entry is free.
- Must not is a ban. Free entry means payment is unnecessary, so do not have to is correct.
- Wrong: Yesterday I must finish my homework. Right: Yesterday I had to finish my homework.
- Must has no past form, so the past obligation uses had to.
- Wrong: You have to be joking; that story is impossible. Right: You must be joking; that story is impossible.
- A deduction about what is certain takes must, not have to.
Practice drill
Answer all 20 questions. Each answer is marked at once, and the rule is explained under the question.
Passengers ___ show their tickets at the gate.
Show answer and explanation
Answer: A. must A written rule from an authority takes must, followed by the base verb show.
I ___ wear a uniform at work; the company requires it.
Show answer and explanation
Answer: B. have to The obligation comes from an external rule, so have to is correct.
You ___ smoke in the hospital. It is forbidden.
Show answer and explanation
Answer: C. must not A prohibition takes must not, which bans the action completely.
You ___ bring a towel; the hotel provides one.
Show answer and explanation
Answer: D. do not have to The towel is unnecessary but allowed, so do not have to expresses no obligation.
She ___ work last Saturday.
Show answer and explanation
Answer: A. had to Must has no past form, so the past obligation uses had to.
We ___ leave early tomorrow if the weather is bad.
Show answer and explanation
Answer: B. will have to The future obligation takes will have to, because must cannot carry will.
You ___ be tired after such a long flight.
Show answer and explanation
Answer: C. must Must expresses a logical deduction about the present, not an obligation.
___ I have to sign the form today?
Show answer and explanation
Answer: D. Do Questions about obligation use do with have to: Do I have to sign?
Children ___ wear helmets on the building site.
Show answer and explanation
Answer: A. have to A general external rule takes have to, and children is plural so have is correct.
He ___ have forgotten our appointment; he is never late.
Show answer and explanation
Answer: B. must Past deduction uses must have plus the past participle forgotten.
I ___ call my mother tonight; I promised her.
Show answer and explanation
Answer: C. have to A personal promise creates an obligation that can take have to in everyday speech.
The notice says guests ___ not use the pool after ten.
Show answer and explanation
Answer: D. must A formal prohibition on the notice takes must not.
You ___ pay for the museum; admission is free.
Show answer and explanation
Answer: A. do not have to Free admission means payment is not necessary, so do not have to is correct.
She ___ to take her medicine every morning.
Show answer and explanation
Answer: B. has The external obligation takes has to with the third-person subject she.
We ___ hurry; the film starts in ten minutes.
Show answer and explanation
Answer: C. must The speaker feels the urgency, so must plus the base verb hurry is correct.
They ___ work on Sunday last week.
Show answer and explanation
Answer: D. had to Last week is a past time, so the past obligation takes had to.
Do we ___ book a table for six people?
Show answer and explanation
Answer: A. have to With the auxiliary do, the obligation uses have to: do we have to book.
That story cannot be true. She ___ be lying.
Show answer and explanation
Answer: B. must Must be plus -ing expresses a strong deduction about what is happening now.
You ___ bring your own lunch; the cafe is open.
Show answer and explanation
Answer: C. do not have to The cafe makes a packed lunch unnecessary, so do not have to fits.
All staff ___ complete the training by Friday.
Show answer and explanation
Answer: D. have to All staff is plural, so the external obligation uses have to.
Drill complete
You answered every question.
Next topic: May vs might: possibility and permissionTake the level test
Questions learners ask
What is the simplest difference between must and have to?
Ask where the obligation comes from. If it comes from the speaker's own feeling or authority, use must. If it comes from a rule, a law or the situation, use have to. In many sentences both are possible, and the choice only changes the flavour.
Is "mustn't" the same as "don't have to"?
No, and this is the most dangerous pair on the page. Must not forbids an action: you cannot do it. Do not have to removes the necessity: you can do it, but nobody requires it. The meanings sit almost opposite each other.
Why does "must" sometimes mean certainty?
Must has two jobs. One is obligation; the other is deduction from evidence. "You must be tired" does not order you to be tired; it says the evidence points to that conclusion. The context or the following verb form shows which job is active.
More grammar topics
May vs might: possibility and permission
May sounds a little more likely and a little more formal. Might sounds more remote and tentative. Both never take to.
Phrasal verbs with up: finish, increase, arrive
The particle up often means completely, higher or appearing. Learn the verbs in meaning groups instead of one long list.
Separable or inseparable phrasal verbs
A noun object can often move, but a pronoun cannot. It must sit between the verb and the particle in a separable phrasal verb.
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