Grammar / C1 Grammar lessons and exercises / Modals: Probability
We use modal verbs and related expressions to show how certain or uncertain we are about a present or future situation.
She might well be on the train.
That must be his dad.
That can't be right.
You're bound to make mistakes occasionally.
We use different structures when we feel very sure that something is true or impossible.
We use must when we feel sure that something is true or when it seems to be 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.
Important
Must here means we reached a conclusion from evidence or reasoning.
We use be bound to to express a very certain guess or prediction.
There are transport strikes tomorrow, so travel is bound to be more difficult.
They're by far the best team – they're bound to win.
Be bound to is not a modal verb, but it expresses a similar idea of certainty.
We use can't when we feel sure that something is impossible.
That can't be right, can it?
It can't be easy for him, looking after three kids on his own.
For probability, use can't, not mustn't, to mean "I'm sure this is impossible".
We use should and shouldn't for expectations about the future when something is 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.
Meaning: if all goes well
We don't normally use this probability meaning of should/shouldn't to predict an unwanted negative event.
We use might, may or could when we think something is possible but we are not sure.
I might see you tomorrow.
There may be another issue.
This illness could be prevented.
They have similar meanings, but may is more formal than might and could.
Adding well after the modal makes the possibility sound more likely.
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. They mean something is possible, but perhaps it will not happen.
We may not need waterproof jackets.
Safi might not come today.
Couldn't is different. In this probability meaning, it means something is impossible.
We do not normally use can for the possibility of one specific situation.
❌ Azi can be in the garden.
✅ Azi may / might / could be in the garden.
Can is used for general possibility – something that sometimes happens or is capable of happening.
Noisy neighbours can be a problem.
It can be very cold here in winter.
modal + infinitive without "to"
✅ She might be there.
❌ She might to be there.
| Form | Meaning | Example |
|---|---|---|
| must | very certain | He must be home. |
| be bound to | very likely / certain prediction | They're bound to win. |
| can't | impossible | That can't be true. |
| should | expected / probable | It should be fine. |
| may / might / could | possible | She might come. |
| can | general possibility | Winters can be cold. |
❌ She might to come.
✅ She might come.
❌ He mustn't be far away. (= deduction)
✅ He can't be far away.
❌ She can be in her office right now.
✅ She may / might / could be in her office right now.