Advanced grammar of modals

Obligation and necessity

A

Must/mustn’t + infinitive without to

Must is used:

1 for strong obligations imposed by the speaker. The speaker uses muse to express his/her authority.

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

2 to give strong advice.

It's a great film. You really must go and see it.

3 to tell oneself what is necessary.

I must remember to phone Roger.

Must not or mustn't is used:

4 to talk about something that is not permitted.

Passengers must not smoke on the aircraft.

You mustn't drive without your seatbelt on in Britain.

5 to give strong advice.

You mustn't work too hard. You'll make yourself ill.

Past form of must

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

We had to write a formal letter in the exam.

Question form with must and have to

Must is possible in question forms:

Must you wear that horrible dress?

although have to is more common:

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.

I have to be at work by 8 o'clock. The boss will get angry if I'm late. (employee to a friend)

Don't have to expresses a lack of obligation.

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

C

The usage of Need to / Don't need to / Needn't

Need to is used to express necessity.

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.

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.

You ought to/should see a doctor about your backache. If you're on a diet you shouldn't drink beer.

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.

Come on, its 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.

You'd better ask your dad before you borrow the car.

Modals for permission

To express permission it is possible to use can, may (more formal) or be allowed to. In the negative these express lack of permission, or prohibition.

You can order another drink but you can't have any more chips.

We aren't allowed to wear trainers to school.

May for past

May is not possible in the past. Could and be allowed to are possible for general permission.

In my last job we had flextime so we could arrive more or less when we wanted to.

Could for particular situation in the past

Could is not used when referring to a particular situation in the past. Only be allowed to is possible.

I was allowed to stay up late last night.

Let and make usage

Both these verbs are followed by the infinitive without to.

Let is used to express permission.

My dad never lets me watch that programme. 

Let is not normally used in the passive. Be allowed to is used instead.

I wasn't allowed to go to the party alone.

Make is used to express obligation.

The teacher made her do some extra homework.

make in passive

In the passive, make is followed by the infinitive with to.

He was made to pay for the window he had broken.

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