Modals: probability

Do you know how to use modal verbs and other expressions to show how probable you think something is? Test what you know with interactive exercises and read the explanation to help you.

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.

Grammar test 1: Grammar test 1: Grammar C1: Modals – probability: 1

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.

Grammar test 2: Grammar test 2: Grammar C1: Modals – probability: 2

Language level

C1 Advanced

Both exersices are very nice👍

Liked both the tests. Few of the answers made me to ponder for a while, wherein both the choices appeared correct, but commonality & set patterns of the sentences and not just the technicalities of the English grammar, it seems, are put forth. I do appreciate.

Thank you for your feedback.

Hello eveyone,

I got 8 correct out of 8 in both Grammar test 1 and Grammar test 2.

That said, I know I have not become perfect at this topic. There is still a lot to learn, study and read.

Shetu Yogme

In these sentences both variants are grammatically correct, but they have different levels of certainty and tone based on the modal verb used (could vs. must). Here's the difference:

You never know. This could be a really good opportunity."
- "Could" expresses possibility.
- This sentence suggests that the opportunity has potential, but you're not certain. It leaves room for doubt and is more exploratory in tone.
"You never know. This must be a really good opportunity."
- "Must" expresses strong certainty or assumption based on some evidence or reasoning.
- This sentence implies that the speaker is confident that this opportunity is indeed good, perhaps based on something they've noticed or concluded.
The doctor is with another patient, but she should not be long."
- "Should not" expresses expectation or likelihood.
- This sentence suggests that the doctor is expected to finish soon, but it's not a guarantee. It’s polite and non-committal.
"The doctor is with another patient, but she must not be long."
- "Must not" implies strong certainty or a requirement.
- This sentence suggests the speaker is either very confident or firmly stating that the doctor is required to finish quickly. It can come across as more assertive or demanding.
I'll check with Alicia, but moving the meeting should not be a problem."
- "Should not" expresses likelihood or expectation.
- This sentence implies that moving the meeting is probably fine, but you're not entirely certain and need to confirm with Alicia.
"I'll check with Alicia, but moving the meeting must not be a problem."
- "Must not" expresses strong insistence or a strict rule.
- This sentence suggests that moving the meeting is absolutely not allowed, or it cannot cause any issues. It comes across as more demanding or authoritative.

Dear team,

I have two Qs below. Thank you very much!

For test 2,

I _____ come to class next week as I have an important exam the day after.

--> "might not" is the answer, but I thought "couldn't" is also correct as it means I am occupied with studies (to study for an exam), so I couldn't attend the class next week. Why is it not the case?

2. He was here a second ago. He _____ be far away.

--> "can't" is the answer, but I thought we could also use "mustn't" as we reach this conclusion by reasoning. Why couldn't we use "mustn't"?

Hello _Yan_,

1. Modal verbs can have multiple uses and sometimes the uses are different with positive and negative forms. For example, it is possible to use could to show probability:

Look at those clouds! It could/might/may rain any moment.
Paula worked hard so she could/may/might pass the exam.

If we change these to require a negative then we find a change in the modals we can use:

It looks like the clouds are moving away. It might not/may not rain after all.
Paula is smart and she worked hard but she's feeling sick today so she may not/might not pass the exam.

What this shows is that we can use could to show probability (a chance of something happening) but we don't use could not for this meaning. We use could not to show that something is impossible or extremely unlikely (there is no/virtually no chance of something happening). This is why couldn't is not possible in the question in the task question.

2. This is similar to the first question in that we use must to show strong belief (I'm sure it is true) but we don't use mustn't to show the opposite. Instead, we use can't or couldn't.

Here is the list of modal verbs we use for logical deduction:

must (very strong belief in something)

could/might/may (something is plausible but not sure)

can't/couldn't (very strong belief against something)

Peter

The LearnEnglish Team

For Question 7 of the 2nd test, I chose "mustn't" as the answer. Although in "Show Answers", it has "can't" as the sole correct answer, I input both answers, including the sentences they are situated in, into Bing Chat, which found that both answers were grammatically correct, because according to Bing,
1. “He was here a second ago.”: This is a complete sentence. It has a subject (“He”) and a verb (“was”). The phrase “a second ago” provides additional context about the timing of his presence.
2. “He mustn’t be far away.”: This is also a complete sentence. The subject is “He,” and the modal verb “mustn’t” indicates a strong probability or necessity. The phrase “far away” describes the distance.
So, are the answers correct or is Bing correct?

Hello ivanskcheung,

This is a good example of the limitations of certain types of language models. The sentence is structurally correct but does not make sense in terms of meaning. The speaker is making a logical deduction: since he was here such a short time ago, it is impossible for him to be very far away now. We do not use mustn't for logical deduction and it does not fit the context here.

Peter

The LearnEnglish Team

If you serch this questions in the OPENAI, It will give you a satisfactory explicit answer.

‘Can't’ indicates a strong sense of impossibility or logical exclusion. Since he was just here, he can’t be far away.

‘Mustn't’ indicates prohibition or something that is not allowed, and it is not suitable for expressing logical possibility in this context.”

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