A. Must / Must not + infinitive without to

Must is used:

1. For strong obligation imposed by the speaker. The speaker uses must to express his/her authority.

Ex: You must be here by 8 am. (manager to employee)

2. to give strong advice.

Ex: It’s a great film. You really must go and see it.

3. to tell oneself what is necessary.

Ex: I must remember to phone Roger.

“Must not”  is used:

4. to talk about something that is not permitted.

Ex: Passengers must not smoke on the aircraft.

Ex: You mustn’t drive without your seatbelt on in Britain.

5. to give strong advice.

Ex: You mustn’t work to hard. You’ll make yourself ill.

Past form

Must does not have a past form. Had to is therefore used to refer to the past.

Ex: We had to write a formal letter in the exam.

Question form

Must is possible in question forms:

Ex: Must you wear that horrible dress?

Although “have to” is more common:

Ex: What do we have to do for homework?

B. Have to / Don’t have to:

“Have to” is used to refer to strong obligations imposed by another person rather than by the speaker or writer.

Ex: I have to be at work by 8 o’clock. The boss will get angry if I’m late. (employee toa friend)

“Don’t have to” expresses a lack of obligation.

Ex: I’m glad I don’t have to wear a suit. It’s so hot today.

C. Need to / Don’t need to / needn’t

“Need to” is used to express necessity.

Ex: Can we go to the baker’s next? I need to get some bread.

Don’t need to / needn’t express a lack of necessity.

Ex: We don’t need to / needn’t leave yet. It’s only 2 o’clock.

D. Should / shouldn’t + infinitive without to

“Should” and “shouldn’t” are used to express obligation or give advice. “Ought to” can also be used with the same meaning as should.

Ex: You ought to/should see a doctor about your backache.

Ex: If you’re on a diet you shouldn’t drink bear.

E. Be supposed to / had better:

“Be supposed to” is used to talk about what you should do because of a rule or because it is expected.

Ex: Come on, it’s 10 o’clock. You’re supposed to be in bed.

“Had better + infinitive” without “to” is used to talk about what you should do because you think it’s a good idea.

Ex: You’d better ask your dad before you borrow the car.

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