Modal verbs: obligations, negative and questions

 

In negative statements ‘must’ and ‘have to’ are used differently.

 

‘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 (base form without ‘to’):

 

Singular	Plural
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 must not smoke in the office.

 

‘Do not have to’ is used when there is no need to do something.

 

We form negative statements with ‘do not have to’ like this:

 

Subject + do not / don’t / doesn’t have to + the verb (base form of the verb):

 

Singular	Plural
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.

 

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 + infinitive form of the verb:

 

Singular	Plural
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‘.