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.
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.
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.