Negative sentences with “must” and “should”

What’s new everyone?

As we have mentioned in previous blog posts, “must” and “should” serve for obligations and suggestions/advice. In today’s blog post, we are going to look at their structure in negative form and when we should be looking to use them in our own English.

How and when do we use “must” in negative?

The use of “must” in the negative form does not vary hugely from that in affirmative. We use the negative form when we are explaining that there is an obligation in place (generally put in place by someone else) which cannot be broken.


Subject + must + not + verb


I mustn’t leave before they get home from work.

They mustn’t say anything to their friends about the project.

You mustn’t tell the secret to anyone.

As can be seen from these examples, there has been a contraction of “must” and “not” to create “mustn’t”. This is done because it is very common to use contractions in English and much more natural for native English speakers and something we should aim towards when studying English. Saying or spelling this without the contraction is still 100% correct.

How and when do we use “should” in the negative form?

Like “must”, the negative form of “should” does not change a great deal either. Its negative form is used when we are explaining that it is not a “well advised” to do something.


Subject + should + not + verb


You shouldn’t go to the concert because you have a lot of homework.

He shouldn’t pay for dinner because he doesn’t have a lot of money.

They shouldn’t go out because they are unwell.

Like with “must” previously, “should” in the negative form can also adopt the contraction. The contraction is very common and natural among native English speakers although it is still 100% correct to say this without using it.

What are the key point to remember?

Like most grammar points, trying to memorise every eventuality in which we could use “must” and “should” in their negative forms is a little unlikely. It is recommended to familiarise yourself with the situations in which these can be used and go from there.

Remember, the more you practise using these in your English, the easier they will become for you when speaking. Sign up for free to ABA English and check out unit 56 for more handy hints and tips.

Good luck

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