Modals: probability

Look at these examples to see how modal verbs and other expressions are used to show how certain the speaker is about something. 

She might well be on the train.
That must be his dad.
That can't be right.
You're bound to make mistakes occasionally.

This page focuses on expressing the probability of present and future situations. To learn how to express probability in relation to past events go to Modals – deductions about the past.
 
Try this exercise to test your grammar.

Read the explanation to learn more.

Grammar explanation

Certainty

must

We use must when we feel sure that something is true or think it's the only realistic possibility.

You must be tired. You've been travelling all day.
That must be Clare's house. I can see her car outside.

Note that we use must to show that we deduced this – we have arrived at this conclusion by reasoning.  

This must be her house. (I've arrived at this conclusion by reasoning or looking at the evidence.)
This is her house. (A simple statement of fact.)

be bound to

We can also use be bound to to express certainty about a guess or prediction.

There are transport strikes tomorrow, so travel is bound to be more difficult.
They are by far the best team – they're bound to win.

Be bound to is not a modal verb, but it is a related expression.

can't

We use can't when we feel sure that something is not possible.

That can't be right, can it?
It can't be easy for him, looking after three kids on his own.

Degrees of uncertainty

should/shouldn't

We use should and shouldn't to show expectations about the future. They show we think something will probably be the case because it's normal or reasonable to expect. 

Rest and drink plenty of water. You should feel better in a day or two.
It shouldn't be a problem.

Should and shouldn't in this context have the meaning of 'if all goes well'. We don't use them to predict something negative or unwanted. 

The treatment should be very painful.
Our flight shouldn't arrive on time.

might, may, could

We use might, may or could to say that we think something in the present or future is possible but we're not sure. 

I might see you tomorrow if you're in the office.
There may be another issue that we don't know about.
This illness could be prevented.

They all have the same meaning, but may is more formal than might and could.

Adding well after the modal makes the situation sound more likely.

I'll try the pharmacy, but it might well be closed now.
She may well have to rethink her plan.
That could well be true.

The negative forms are may not and might not (or mightn't).

We may not need waterproof jackets, but I'll pack them anyway.
Safi might not come today as he has his driving test.

Couldn't is different from may not and might not. It means something is impossible.

General possibility

can

Note that can is not used to talk about possibility in relation to a specific event or situation. 

Azi can may/might/could be in the garden.

Instead, can shows that something sometimes happens or is capable of happening.

Noisy neighbours can be a problem if you're living in a flat.
It can be very cold here in winter.

Note that all modal verbs are followed by an infinitive without to.

Do this exercise to test your grammar again.