Main points

* You use `must’, `ought’, `should’, or `will’ to express probability or certainty.

* You use `cannot’ or `can’t’ as the negative of `must’, rather than `must not’ or `mustn’t’, to say that something is not probable or is not certain.

1 When you want to say that something is probably true or that it will probably happen, you use `should’ or `ought’. `Should’ is followed by the base form of a verb. `Ought’ is followed by a `to’-infinitive.
We should arrive by dinner time.
She ought to know.

When you want to say that you think something is probably not true or that it will probably not happen, you use `should not’ or `ought not’.
There shouldn’t be any problem.
That ought not to be too difficult.

2 When you want to say that you are fairly sure that something has happened, you use `should have’ or `ought to have’, followed by a past participle.
You should have heard by now that I’m leaving.
They ought to have arrived yesterday.

When you want to say that you do not think that something has happened, you use `should not have’ or `ought not to have’, followed by a past participle.
You shouldn’t have had any difficulty in getting there.
This ought not to have been a problem.

3 You also use `should have’ or `ought to have’ to say that you expected something to happen, but that it did not happen.
Yesterday should have been the start of the soccer season.
She ought to have been home by now.

Note that you do not normally use the negative forms with this meaning.

4 When you are fairly sure that something is the case, you use `must’.
Oh, you must be Sylvia’s husband.
He must know something about it.

If you are fairly sure that something is not the case, you use `cannot’ or `can’t’.
This cannot be the whole story.
He can’t be very old – he’s about 25, isn’t he?

WARNING: You do not use `must not’ or `mustn’t’ with this meaning.

5 When you want to say that you are almost certain that something has happened, you use `must have’, followed by a past participle.
This article must have been written by a woman.
We must have taken the wrong road.

To say that you do not think that something has happened, you use `can’t have’, followed by a past participle.
You can’t have forgotten me.
He can’t have said that.

6 You use `will’ or `-‘ll’ to say that something is certain to happen in the future.
People will always say the things you want to hear.
They’ll manage.

You use `will not’ or `won’t’ to say that something is certain not to happen.
You won’t get much sympathy from them.

7 There are several ways of talking about probability and certainty without using modals. For example, you can use:

* `bound to’ followed by the base form of a verb
It was bound to happen.
You’re bound to make a mistake.

* an adjective such as `certain’, `likely’, `sure’, or `unlikely’, followed by a `to’-infinitive clause or a `that’-clause
They were certain that you were defeated.
I am not likely to forget it.
See Unit 33 for more information on these adjectives.