Unit 20:

Grammar

Modals 2: obligation, probability, possibility

For general information about modals, see Unit 19.

Obligation

Use

Modal

Example

Present of future obligation

must/ mustn’t

have to

Need to

All visitors must turn off their mobile phones.

You have to/ need to press “send”.

No present or future obligation

don’t have to

You don’t have to/ don’t need to/ needn’t pay to

send an e-mail.

Past obligation

had to

Yesterday, Sam had to buy more stamps.

No past obligation

didn’t have to

didn’t need to

I learnt a little Italian, but everyone spoke

English, so I didn’t have to/ didn’t need to use it.

Helpful hints

In spoken English, have to is more common that must. Must is often used in written notices and

instructions.



‘We have to pay the phone bill today,’ Rita said.



Passengers must turn off all mobile phones.

Watch out

Mustn’t and don’t have to do not mean the same.



You mustn’t do that! (= Don’t do that)



You don’t have to do that. (= You can do that if you want to, but it’s not necessary.)

* Probability and possibility

Use

Modal

Example

Present strong probability

must

The phone is ringing – it must be Simon.

can't

couldn’t

This letter can’t/ couldn’t be from Japan because

it’s got a French stamp

Present and future probability

should

ought to

We ought to/ should hear from Cheryl this

weekend.

Present and future possibility

could

may

might

I’m not sure what language it is – it could/ may/

might be Polish.

* Helpful hints

We often use must, can’t and couldn’t for probability when we have some evidence for our opinion.



I just rang Paul, but there’s no answer. He must be out.

Watch out

To talk about possibility and probability about the past, we use a modal and the perfect infinitive.

See Unit 22.

A. Choose the sentence (A, B or C) which means the same as the first sentence.

1. We have to pay the electricity bill before Friday.

5. Students mustn’t run in school buildings.

1