have to, must, should – obligation, prohibition, necessity, advice

Must/mustn’t, have to/don’t have to – Form

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Have to

Have to is NOT an auxiliary verb. We need do/does/did to make negatives and questions in the present and past.

• Have you to get up early tomorrow?
• Do you have to get up early tomorrow?
• Did you have to change the sound card?
• We don’t have to stay until the end.

We can use have to in all the verb forms (present, past and perfect tenses, gerund, to infinitive, etc.)

• I don’t like having to get up early.
• She’s had to work at weekends since she started working.
• I had to cancel the meeting.

It is used to describe an obligation, a rule, or something that is necessary.

• You have to drive on the left in England.
• I’ll have to get up early tomorrow. The exam is at 8.

Have got to

In spoken informal English, you can often use ‘ve got to instead of have to.

• I can’t stay; I’ve got to go to the supermarket.

Must

Must is a modal verb, and it has a present form, which can be used to talk about the present or future. The negative form is must not or mustn’t, and the question is must I, must you, etc.

• I must go to the doctor.
• I must get up early tomorrow.
• You mustn’t call me before 8.

It is used to describe something that the speaker thinks is necessary to do.

• You must eat less candy.
• I must try to do more exercise.

We can also use must to express strong advice.

• You must see the new film; it’s amazing!

Must/mustn’t, have to/don’t have to – Use

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Must or Have to?

Must only has a present form, so we need to use have to for all other verb forms (past, future, perfect forms, infinitive, etc.).

• You will have to come with me.
• We had to drive very fast.

We normally use have to for questions. Questions with must are not common.

• Do we have to sit here?

Regarding the meaning, sometimes there is little difference between have to and must.

• I must go to the hospital.
• I have to go to the hospital.

But we normally use have to when there’s an external obligation and must when the obligation comes from the opinion of the speaker.

• We have to get up early tomorrow. The meeting is at nine. (external obligation)
• We must get up early tomorrow if we want to finish painting. (it’s the speaker’s opinion)
• You have to wear a suit at the meeting. (it’s a rule in the company)
• You must buy a new suit for the meeting. (it’s my opinion)

Mustn’t or Don’t have to?

Don’t have to and mustn’t have opposite meanings. We use don’t have to when we don’t need to do something, when there’s no obligation, and we use mustn’t to talk about prohibition, when there is obligation not to do something.

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

If you don’t have to do something, it means that you can do it if you want, but you don’t need to do it.

Can’t/be not allowed to = mustn’t

We can use can’t or be not allowed to  instead of mustn’t.

• You mustn’t smoke in this area. = You are not allowed to/can’t smoke in this area.
• Children mustn’t eat chewing gum. = Children can’t/are not allowed to eat chewing gum.

Should, shouldn’t

Should is used to give advice or an opinion about what we think is right or wrong.

• You should go to a therapist.
• I think schools shouldn’t offer soft drinks to their students. 

Should is not as strong as must or have to.

• You should be patient with me. (=advice)
• You must be patient with me. (=strong advice)

Ought to/ought not to = should/shouldn’t

Ought to has the same meaning as should, although it is more formal and not as common.

• You ought to go to a therapist.
• You ought not to be so strict with your daughter

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