Must and Have To: Obligation, Prohibition, and Choice

Must and have to both express necessity: “You must show your ticket” and “You have to show your ticket” can describe the same requirement. Their negative forms are very different, however.

“You mustn’t leave” means leaving is not allowed. “You don’t have to leave” means leaving is not necessary, so you have a choice. Understanding that difference is more useful than treating every use of must as a personal rule and every use of have to as an external rule.

Compare four meanings in one situation

Imagine you are attending a training session. The organiser gives you these instructions, each with a different purpose.

The last instruction does not forbid laptops. Replacing don’t have to with mustn’t would change the rule, rather than simply make it stronger.

Forming sentences with must

Use must + base verb for every subject. There is no third-person -s and no to before the next verb: “She must arrive early,” not “She musts to arrive early.”

For the negative, use must not or mustn’t. A question puts must before the subject, although Do I have to…? is often a more usual way of asking about an everyday requirement.

Must often gives a strong instruction, a rule, or something the speaker considers necessary. It can also introduce an enthusiastic recommendation: “You must try this soup.” Context tells you whether there is a real obligation.

Forming sentences with have to

Unlike must, have to changes with the subject and tense. Use has to with he, she, and it in affirmative present sentences, but use have to after does or doesn’t.

For an obligation in the past, use had to: “Yesterday I had to work late.” Do not use must as though it were a past-tense form. “He must have worked late” has a different meaning: the speaker is drawing a conclusion about what probably happened.

How to choose between must and have to

Have to is common when explaining practical requirements: “I have to catch the last bus.” Must can make the speaker’s insistence more noticeable: “I must remember to call Dad.” These are tendencies, not a strict division between outside and inside obligations.

Rules can use either form: “Visitors must sign in” and “Visitors have to sign in” both describe a requirement. Choose wording that fits the situation, and pay particular attention to the negative meaning.

At work

Employee: Do I have to attend the meeting in person?
Manager: No, you don’t have to. You can join online.
Employee: Can I share the meeting link?
Manager: You mustn’t share it outside the team.

The first answer gives an alternative. The second sets a restriction, so don’t have to would not express the manager’s meaning.

At school

Student: Did we have to finish the whole chapter?
Teacher: No, you only had to read the first two pages.
Student: Do we have to answer every question?
Teacher: Yes, but you don’t have to write long answers.

After did, use have, not had. “You didn’t have to read the whole chapter” tells us what was unnecessary; it does not establish whether the student actually read it.

Common mistakes

For more examples of the central negative contrast, see Cambridge’s explanation of must. Practise the two meanings in a real situation, such as a classroom rule or an optional work event.

Practice: required, forbidden, or optional?

Choose the expression that matches the information. Where the task asks for a particular tense, include the necessary auxiliary verb.

1. Photographs are forbidden. You _____ take photographs. (mustn’t / don’t have to)
2. Breakfast is included, but eating it is optional. You _____ have breakfast. (mustn’t / don’t have to)
3. Complete the question: “_____ she have to work on Saturdays?”
4. Rewrite in the past: “We have to take a taxi.”
5. Correct: “He doesn’t has to wear a tie.”
6. The office will be closed, so tomorrow we _____ work from home. (will have to / had to)
7. Does “I didn’t have to pay” necessarily mean that I paid nothing?
8. Rewrite as a strong rule: “It is necessary for everyone to switch off their phone.” Use must.

Answers and explanations

1. Mustn’t. The action is prohibited.
2. Don’t have to. The action is optional.
3. Does. The complete question is “Does she have to work on Saturdays?”
4. We had to take a taxi. Had to expresses past necessity.
5. He doesn’t have to wear a tie. Use have after doesn’t.
6. Will have to. The requirement applies tomorrow.
7. No. It says payment was unnecessary. Context would tell us whether any payment was made voluntarily.
8. Everyone must switch off their phone. Must stays the same after every subject.