Modal Verbs – Must and Have to

Must and Have to are modal verbs that show necessity, obligation and prohibition. They have little differences in their usage.

‘Must’ is used to imply that the obligation is personal. It is an obligation the speaker sets for himself. We can only use this form in the present and in the future.

Basic sentence structure:

subject + auxiliary verb must + main verb base

Examples:

I must finish this task. (Finishing the task is an obligation the speaker took on himself)

I must go.  (obligation set by the speaker)

I must learn how to drive. (obligation set by the speaker)

‘Have to’ also implies obligation. However, this obligation is something external. It is something someone else asked the speaker to do and the speaker cannot change it. We can use this form in the past, the present and the future.

Basic sentence structure:

subject + auxiliary verb + have + to-infinitive

Examples:

I have to send the report as soon as possible. (Sending the report is a duty given to the speaker.)

They had to stay there longer. (They were required to stay longer)

I have to leave now. (Leaving now is required of the speaker)

For the question form, it is more common to use “have to’ than ‘must’. It is used to ask if something is required or an obligation.

Why do you have to go?    – more common

Why must you go?             – less common

Does he have to drive?      – more common

Must he drive?                    – less common

What do we have to say?   – more common

What must we say?            – less common

The negative forms of these two have different meanings.

The negative form of ‘must’ is ‘must not’. It shows that something is not allowed.

He mustn’t sleep at work.

You must not leave the house.

They must not stay outside.

The negative form of ‘have to’ is ‘don’t have to’. It shows that something is not necessary but you can do it if you want to.

You don’t have to attend the meeting.

They didn’t have to join the club.

She doesn’t have to wear the dress.