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English Grammar: when to use ‘must’ and ‘mustn’t’

Many learners confuse how to use the words ‘must’ and ‘mustn’t’ in English. While they might seem tricky, they are actually quite simple to use. ‘Must’ is a modal verb, which means it does not show an action like most verbs. Instead, we use it to show two things, necessity or possibility.


We can use ‘must’ in a sentence to talk about something we need to do. For example, ‘I must get my hair cut before my meeting tomorrow’. In this sentence, we can see it is important for me to get my hair cut before the meeting. Maybe I want to make a good impression on my boss!

The opposite is ‘must not’ or ‘mustn’t’. We use this to talk about thinks we need to avoid doing. For example ‘I mustn’t drink too much beer at this party’. In this sentence, we can see that I don’t think it is a good idea to drink too much beer at the party and want to avoid doing it. This might be because of my important meeting with my boss.

There are other modal verbs for talking about necessity, such as ‘have to’. Usually when we use the word ‘must’ it is because the speaker has decided something is necessary to do. In the example above, the speaker decided it would be a good idea to cut his hair. When we use ‘have to’, it is often because someone else has decided the action is necessary. For example ‘I have to cut my hair because long hair isn’t allowed at my new company’.

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When we are talking about possible causes of a situation or action, we can use the word ‘must’ to show we are fairly sure of the reason behind something. For example:

The cookies are missing. Sally must have taken them.

In this example, the speaker is sure that Sally is the person who took the cookies. Maybe Sally loves cookies or has a history of taking food from the kitchen. Whatever the reason, the speaker has decided that it was Sally who took the cookies.

In the opposite situation, we use ‘couldn’t’ or ‘can’t’ instead of ‘mustn’t’. For example:

The cookies are missing. Jayne couldn’t have taken them.

In this example, the speaker is sure that Jayne didn’t take the cookies. Perhaps she is on a diet or just doesn’t like them. Either way, the speaker has decided that it’s not possible that Jayne took them.

In this situation, we never use ‘mustn’t’. Using ‘mustn’t’ in this situation is a common problem with English learners so make sure you avoid doing it.

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