What is the Modal Verb Must?
The modal verb 'must' expresses necessity, obligation, rules, and strong requirements. It is the strongest obligation modal. Use it for laws, requirements, duties, and when something is absolutely necessary or prohibited.
Must expresses strong necessity ('I must leave now'), obligation ('You must follow rules'), prohibition ('You mustn't cheat'), and logical certainty ('It must be 3 o'clock').
Step-by-step worked examples
I must leave now or I'll be late.
'Must' shows strong necessity. There's no choice — I have to go.
You must obey the traffic law.
'Must' expresses an obligation or rule. It's mandatory, not optional.
You mustn't cheat on the exam.
'Must not' (mustn't) is a prohibition. It is absolutely forbidden.
Flashcards
Quick quiz
Q1.'You must wear a seatbelt.' This is…
Q2.Which is stronger?
Q3.'You mustn't shout in class.' This means…
Q4.'The answer must be 5.' This suggests…
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Common mistakes
Using 'must' for advice like 'should'. — Correct: 'Must' is obligation/law; 'should' is advice.
'I must to go.' — Correct: 'I must go.' — no 'to' after modal.
Confusing 'mustn't' with 'don't have to'. — Correct: 'Mustn't' = forbidden; 'don't have to' = optional.
Not recognizing 'must' for logical certainty. — Correct: 'It must be 3 PM' means we're almost sure.
FAQ
What is the modal verb must?
Must expresses strong necessity, obligation, rules, prohibition, and logical certainty.
Difference between must and should?
'Must' = requirement/rule; 'should' = advice or recommendation.
What does 'mustn't' mean?
Prohibition — something is absolutely forbidden, not optional.
Can 'must' express certainty?
Yes. 'You must be tired' means we're almost certain you are.