Modal Verbs for Obligations: Negative and Questions

Do you remember which modal verbs we can use to express obligation? To talk about obligation, freedom and necessity to do something, we use the verbs ‘must‘ and ‘have to‘.

In negative statements and questions, ‘must’ and ‘have to’ have special meaning and are used differently.

Remember that modal verbs are always followed by a base form of a verb – an infinitive verb without to.

Negative sentences

Prohibition

Must not’ is a negative obligation, meaning that something is not allowed.

We form negative statements with ‘must not’ like this:

subject + must not + the verb

SingularPlural
I must not (mustn’t) speak
You must not speak
He/she/it must not speak
We must not speak
You must not speak
They must not speak
  • She must not take this medicine.
  • You mustn’t smoke in the office.

No obligation

We use ‘don’t have to to show that there is no obligation. You can do something if you want to but it’s not compulsory.

The structure is:

subject + do not / don’t / doesn’t have to + the verb

SingularPlural
I do not (don’t) have to wait
You don’t have to wait
He/she/it doesn’t have to wait
We don’t have to wait
You don’t have to wait
They don’t have to wait
  • You don’t have to buy me a birthday present.
  • It’s summertime, and Peter doesn’t have to go to school.

Let’s summarize:

'mustn't' vs 'don't have to'
via https://pt.slideshare.net/maypulido/mustnt-blog/6

Questions

We also use ‘have to’ in questions, for example, to ask if something is necessary. Such questions are formed like this:

Do / Does + subject + have to + the verb

SingularPlural
Do I have to speak?
Do you have to speak?
Does he/she/it have to speak?
Do we have to speak?
Do you have to speak?
Do they have to speak?
  • Do I have to get ready now?
  • Does she have to leave before 12 a.m.?
Note: ‘Must’ is possible for questions, for example:

Must we finish this today?

But it is not as common as ‘have to‘.

Watch this video from Woodward English where you’ll learn the difference between ‘mustn’t’ and ‘don’t have to’ and can practise these phrases:

Read more on this topic:

Modal Verbs for Obligations: Statements

Modal Verbs for Necessity (‘Need’)

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