Have to, don’t have to, must, mustn’t

must/mustn’t

Use must + infinitive without to

Must is a modal verb, and modal verbs are followed by infinitive without to.

• I must go to the doctor. (NOT I must to go to the doctor.)

No -s

Modal verbs don’t add -s after he/she/it.

• He must come with us. (NOT He must comes with us.)

Use must/mustn’t in negatives and questions

Must is an auxiliary verb. This means that it has a negative and an interrogative form. Do not use do/did in negatives and questions.

• We mustn’t arrive early. (NOT We don’t must arrive early.)
• Must we arrive early? (NOT Do we must arrive early?)

have to/don’t have to

Use do and did in negatives and questions

The verb have in have to is not an auxiliary verb. Use do/don’t, does/doesn’t, did/didn’t in questions and negative sentences in present and past.

• Do you have to get up early tomorrow? (NOT Have you to get up early tomorrow?)
• Did you have to buy a new sound card?
• We don’t have to stay until the end.

No contraction

The verb have in have to cannot be contracted to ‘ve.

• I have to study for the exam tomorrow. (NOT I’ve to study for the exam tomorrow.)

Past and future forms

Only have to has a past and a future form. Must does not have a past or a future form.

• Yesterday, I had to take a taxi. (NOT Yesterday, I must take a taxi)
• You will have to take a taxi if it rains tomorrow. (NOT You will must take a taxi.)

must vs have to / mustn’t vs don’t have to – meaning

have to

Rules and obligations

We use have to talk about rules and obligations, something that is necessary.

• I have to get to work before 7 tomorrow.
• The car broke down and we had to call a taxi.

must

Rules and obligations

Must is very similar to have to. We use must + infinitive (without to) to talk about rules and obligations.

• I must go to the doctor.
• I must get up early tomorrow.

don’t have to

Don’t need to – not necessary

If you don’t have to do something, it means that you ‘don’t need to’ do something (there is no obligation). You can do it but you don’t need to do it if you don’t want to do it.

• You don’t have to pick me up at the station. I can take a taxi. (=You can pick me up, but you don’t need to do it).

mustn’t

Prohibition

Use mustn’t to talk about something that you can’t do. It’s necessary that you don’t do it.

• You mustn’t call me before 8. (=You can’t call me before 8.)
• I mustn’t lose my concentration now.

must vs have to

must

Obligation from the speaker

We normally use must when to talk about obligations that come from the opinion of the speaker. The speaker thinks it’s necessary or important to do something.

• MANAGER: You must get up early tomorrow. The meeting is at nine.(=The obligation comes from the manager. The manager thinks it’s necessary.)
• DOCTOR: You must stop smoking. (=It’s the doctor’s opinion. The doctor thinks it’s necessary.)
• SMOKER: I must stop smoking. (=It’s the smoker’s opinion. The smoker thinks it’s necessary to stop smoking.)

have to

External obligation

We normally use have to when there’s an external obligation. The obligation doesn’t come from the speaker’s opinion; another person thinks it’s necessary.

• WORKER: We have to get up early tomorrow. The meeting is at nine. (=The obligation comes from the manager, not from the worker.)
• SMOKER: I have to stop smoking. (=It’s the doctor’s opinion, not the smoker’s opinion. The doctor thinks it’s necessary.)

mustn’t vs don’t have to

don’t have to

You don’t need to do it; not necessary; no obligation

Don’t have to and mustn’t have opposite meanings. We say that we don’t have to do something when we don’t need to do something; we can do it, but it’s not necessary.

• You don’t have to wait here. (=You can do it, but it’s not necessary)

mustn’t

Prohibition; it’s necessary that you don’t do it

We say that we mustn’t do something when we cannot do something; it is necessary that we don’t do something.

• You mustn’t wait here. (=You cannot do it; it’s against the rules)

Exercises on the theme: