OBLIGATION & NECESSITY

+ infinitive without to

Must / Must not

Obligation and Necessity

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

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

"Must" is used:

Ex: I must remember to phone Roger.

3. to tell oneself what is necessary.

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

2. to give strong advice.

Ex: Passengers must not smoke on the aircraft.

4. to talk about something that is not permitted.

“Must not” is used:

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

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

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

5. to give strong advice.