Modal verb
The modal verb must
must · never changes
A2Meaning: to be obliged to do something; to be almost sure · Type: modal
Key points to remember
- Modal verb of strong obligation
- Same form for everyone:
I/you/he/she/we/they must - No past form: in the past, use had to
- Negative:
must notormustn't= it is not allowed
Conjugation of the modal verb must
Modal verbs never change: the same form for I / you / he / she / it / we / they, no -s in the 3rd person, and no to before the next verb.
Affirmative
subject + must + base form
- You must wear a uniform.
- I must finish my homework.
Negative
subject + must not / mustn't
- You mustn't smoke here.
- We mustn't be late.
Question
must + subject + base form?
- Must I come?
- Must we wait?
No past form: use had to instead. Mustn't = it is not allowed; don't have to = it is not necessary (don't confuse them).
How to use must
1. Strong obligation (from you or from others)
- You must wear a uniform.
- I must finish my homework.
2. Being almost sure (logic)
You must be tired! means I'm almost sure you are tired.
- You must be tired!
- She must be home by now.
3. Not allowed (mustn't)
- You mustn't smoke here.
Common mistakes to avoid
1. I musted ❌
Must has no past form. In the past, use had to: I had to leave. Never I musted or I mussed.
2. Mustn't ≠ don't have to
Mustn't = it is not allowed. Don't have to = it is not necessary (you can choose). You mustn't use your phone in class (not allowed). You don't have to come (you are free).
3. I must to go ❌
I must go ✓ / I must to go ✗. After a modal verb like must, can or should, the next verb has no to. Only have to takes to: I have to go.
Going further
Conjugating to must is a key skill in the English contest
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Related pages: must
Pages on a similar topic or with a similar use, so you can keep exploring.
Frequently asked questions
How do you use must?+
Must shows a strong obligation: You must wear a uniform. It can also show that you are almost sure something is true: You must be tired. After must, use the base form of the verb, with no to.
What is the difference between must and have to?+
Must often shows an obligation that comes from the speaker: you decide it is necessary. Have to often shows an obligation from outside: a rule or another person. I must study tonight (I want to). I have to wear a uniform (the school says so).
What is the difference between mustn't and don't have to?+
Mustn't means it is not allowed: You mustn't run in the corridor. Don't have to means it is not necessary, so you are free: You don't have to come. The two meanings are very different!
What is the past tense of must?+
Must has no past form, so use had to: I had to leave early. For the future, say I will have to, not I will must.
Can must mean that you are sure?+
Yes. Must also shows that you are almost sure, because it is logical: He must be tired (he worked all day, so it is probably true). She must be at home. It is a strong guess.
Updated on 12 November 2026 by the Big Challenge teaching team. Every year, more than 500,000 students across Europe take part in our English contest, created in 1999. Our teaching approach →
The Big Challenge · Updated on 12 November 2026 · Source: www.thebigchallenge.com/bl/academy/learn-english/verbs/must
© The Big Challenge. Copying allowed for classroom use. Every year, more than 500,000 students across Europe take part in our English contest, created in 1999.