must not
/mʌst nɒt/Used to express a strong prohibition or command that something should not be done under any circumstances.
Students must not cheat on exams.
This rule emphasizes that cheating is strictly forbidden in exams.
You must not leave the house without telling me.
The speaker is insisting that the listener must inform them before leaving.
Often used in rules, regulations, or strong warnings. Can be softened with 'should not' for less strict advice.
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💡Pro Tip
Formal vs. Informal Use
'Must not' is formal and strong, while 'should not' is softer. In casual speech, 'mustn't' is common.
⚡Gold Rule
Strong Prohibition
'Must not' is used to enforce rules or express strong disapproval, not just advice.
📖Word Origin
Derived from the modal verb 'must' (expressing obligation) combined with the negation 'not' to form a strong prohibition.
📝Usage Notes
In formal contexts, 'must not' is stronger than 'should not.' In informal speech, 'mustn't' is often used as a contraction.