Expressing Probability in English: May, Might, Will, Must and More

Expressing probability in English means showing how sure you are that something is true, possible or likely to happen. This is an important skill in everyday conversation, writing, exams and professional communication.

Instead of saying only I think or maybe, you can use modal verbs, adverbs and adjectives to express different levels of certainty.

For example:

• It will rain tomorrow.
• It might rain tomorrow.
• It probably won’t rain tomorrow.
• It is unlikely that it will rain tomorrow.

All these sentences talk about the future, but they do not express the same level of certainty.

In this lesson, you will learn how to express probability in English using words such as may, might, could, will, must, can’t, probably, definitely, likely, unlikely and more.

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Why Is Expressing Probability Important?

Good English is not only about saying what you think. It is also about showing how sure you are.

Compare these sentences:

• She is late.
• She might be late.
• She’ll probably be late.
• She must be late.

The basic idea is similar, but the meaning changes depending on the level of certainty.

This is especially useful when you want to:

• make predictions
• give opinions
• write essays or reports
• speculate about a situation
• discuss possibilities
• sound more natural and precise

It is also very useful for Cambridge exams such as B2 First and C1 Advanced, especially in Speaking and Writing.

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A Probability Scale in English

Think of probability as a scale from 0% to 100%.

100% certain	definitely, certainly, will	She’ll definitely pass the exam.
Almost certain	must, be certain, clearly	He must be tired.
Very likely	probably, likely	They’ll probably arrive soon.
Possible	may, might, could, possibly	It might rain later.
Not very likely	probably won’t, unlikely	He probably won’t come.
Almost impossible	can’t, impossible	That can’t be true.

This scale helps you choose the right expression.

The more certain you are, the stronger the word you use.

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Modal Verbs for Expressing Probability

Modal verbs are very common when we talk about probability. They help us say whether something is certain, probable, possible or impossible.

The most useful modal verbs for probability are:

• will
• may
• might
• could
• must
• can’t

Let’s look at them one by one.

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Will for Strong Probability

We use will when we are very confident that something is true or that something is going to happen.

Examples:

• She’ll be here soon.
• Don’t worry. Everything will be fine.

In these sentences, the speaker is not simply guessing. They feel very sure.

We often use will for predictions based on what we know, believe or expect.

• Prices will probably rise next year.
• He’ll definitely call you when he arrives.

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May, Might and Could for Possibility

We use may, might and could when something is possible, but not certain.

• It may rain later.
• It might rain later.
• It could rain later.

All three sentences mean that rain is possible.

May

May often sounds a little more formal.

• The train may be delayed.
• She may decide to apply for the job.
• There may be some changes to the timetable.

Might

Might is very common in everyday English. It sometimes sounds a little less certain than may, but the difference is often small.

• I might stay at home tonight.
• He might know the answer.
• They might not agree with us.

Could

Could often suggests one possible explanation or one possible future result.

• She could be at work.
• The problem could be serious.
• This decision could affect many people.

Compare:

• She must be at work. = I am almost sure.
• She might be at work. = It is possible.
• She could be at work. = That is one possible explanation.

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Must for Strong Deduction

We use must when we are almost sure something is true because we have evidence.

• The lights are on. They must be at home.
• You’ve been studying English for years. You must know a lot of grammar.

In these examples, we are making a logical conclusion.

Important structure:

must + infinitive without to

Correct:

• He must be tired.

Incorrect:

• He must to be tired.

For a more detailed grammar lesson, you can read my lesson on modal verbs of deduction.

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Can’t for Impossibility or Strong Negative Deduction

We use can’t when we are almost sure something is not true or is impossible.

• That can’t be Tom. Tom is much taller.
• She can’t be at home. Her car isn’t outside.
• He can’t be the manager. He’s only twelve.

In this use, can’t means:

I am almost sure this is not true.

Compare:

• He must be at home. = I am almost sure he is at home.
• He can’t be at home. = I am almost sure he is not at home.

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Adverbs of Probability

Adverbs are also very useful for expressing probability in English. They help us show how certain or uncertain we are.

Adverb	Meaning	Example
definitely	100% sure	She’ll definitely call you.
certainly	100% sure	He’ll certainly agree.
probably	very likely	They’ll probably arrive late.
possibly	maybe	We’ll possibly stay another day.
maybe	perhaps	Maybe I’ll stay in tonight.
perhaps	maybe, slightly more formal	Perhaps it’ll snow tomorrow.
clearly	obviously	He’s clearly not ready.

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Word Order with Adverbs of Probability

Word order is very important.

Adverbs such as probably, definitely and certainly usually go after an auxiliary verb or before the main verb.

• She will probably arrive late.
• They will definitely help us.
• He is certainly interested in the job.

With modal verbs, the adverb usually comes after the modal verb.

• She’ll probably come.
• They might possibly cancel the match.
• He’ll definitely pass the exam.

Avoid this structure in normal English:

• Probably she will arrive late.

This is not always grammatically impossible, but it sounds awkward in many everyday contexts. A more natural sentence is:

• She will probably arrive late.

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Maybe vs May Be

This is a very common mistake.

Maybe is one word. It means perhaps.

• Maybe she is at work.
• Maybe they forgot the meeting.

May be is two words: may + be.

• She may be at work.
• They may be busy.

Compare:

• Maybe he is ill.
• He may be ill.

The meaning is similar, but the grammar is different.

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Adjectives of Probability

We can also use adjectives to express probability.

Common adjectives include:

• likely
• unlikely
• certain
• possible
• impossible

The structure is often:

It is + adjective + that + clause

• It is likely that she’ll get the job.
• It is unlikely that we’ll finish today.
• It is possible that they missed the train.
• It is certain that prices will rise.
• It is impossible that he forgot your birthday.

In less formal English, we often leave out that.

• It’s likely she’ll get the job.
• It’s possible they missed the train.

Both forms are common, but It is likely that… is often better in formal writing.

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Likely and Unlikely

Likely means that something will probably happen.

Examples:

• She is likely to pass the exam.
• It is likely that the company will expand.

Unlikely means that something probably will not happen.

• He is unlikely to change his mind.
• It is unlikely that we’ll finish on time.

Useful structures:

be likely to + infinitive

• She is likely to win.
• Prices are likely to rise.

be unlikely to + infinitive

• He is unlikely to come.
• The situation is unlikely to improve quickly.

It is likely/unlikely that + clause

• It is likely that she will win.
• It is unlikely that they will agree.

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Possible and Impossible

We use possible when something can happen.

• It is possible that he missed the train.
• Is it possible to change the date?
• A mistake is possible, but I don’t think it’s likely.

We use impossible when something cannot happen or cannot be true.

• It is impossible to finish this in five minutes.
• It is impossible that she forgot the meeting.
• This explanation sounds impossible.

Be careful:

Possible does not mean probable.

Something can be possible, but not likely.

• It is possible that I’ll win the lottery, but it is not likely.

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Common Phrases for Expressing Probability

Here are some useful phrases you can use in speaking and writing.

When you are very sure

• I’m sure that…
• I’m certain that…
• There is no doubt that…
• It will definitely…
• It is certain that…

Examples:

• I’m sure she’ll do well.
• There is no doubt that technology has changed education.
• It will definitely be useful in the future.

When something is likely

• It will probably…
• It is likely that…
• There is a good chance that…
• The chances are that…

Examples:

• It will probably take longer than expected.
• There is a good chance that he’ll accept the offer.
• The chances are that prices will continue to rise.

When something is possible

• It may…
• It might…
• It could…
• It is possible that…
• There is a possibility that…

Examples:

• It might be difficult at first.
• There is a possibility that the meeting will be cancelled.
• It could become a serious problem.

When something is unlikely

• It probably won’t…
• It is unlikely that…
• There is little chance that…
• I doubt that…

Examples:

• It probably won’t snow tomorrow.
• It is unlikely that everyone will agree.
• I doubt that he’ll change his opinion.

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Expressing Probability in Cambridge Exams

In Cambridge B2 First and C1 Advanced, expressing probability helps you sound more precise and natural.

You can use probability language in:

• essays
• reports
• proposals
• reviews
• speaking tasks
• sentence transformations
• stories

For example, instead of writing:

• This problem is bad.

You could write:

• This problem is likely to become more serious in the future.

Instead of saying:

• Maybe the man is tired.

You could say:

• The man might be tired because he looks exhausted.

Instead of saying:

• I think technology is important.

You could say:

• Technology will probably become even more important in education.

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Common Mistakes with Probability Words

Mistake 1: Using “to” after a modal verb

Incorrect:

• He must to be tired.
• She might to come later.

Correct:

• He must be tired.
• She might come later.

After modal verbs, use the infinitive without to.

Mistake 2: Using “to” after probably

Incorrect:

• She will probably to come.

Correct:

• She will probably come.

Mistake 3: Confusing maybe and may be

Incorrect:

• She maybe at work.

Correct:

• She may be at work.
• Maybe she is at work.

Mistake 4: Forgetting “that” in formal writing

Informal:

• It is likely she will win.

More formal:

• It is likely that she will win.

Both are possible, but the second sentence is better for formal writing.

Mistake 5: Using mustn’t instead of can’t

Incorrect:

• He mustn’t be at home. I’ve just seen him in town.

Correct:

• He can’t be at home. I’ve just seen him in town.

Use can’t when you are sure something is not true.

Use mustn’t for prohibition.

Example:

• You mustn’t park here. It’s not allowed.

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How to Choose the Right Probability Word

Ask yourself one question:

How sure am I?

If you are almost 100% sure, use:

• definitely
• certainly
• will
• must

Examples:

• She’ll definitely pass.
• He must be tired.

If something is very likely, use:

• probably
• likely
• there is a good chance that

Examples:

• They’ll probably arrive soon.
• It is likely that prices will rise.

If something is possible, use:

• may
• might
• could
• possibly
• it is possible that

Examples:

• She might be at home.
• It is possible that the train is delayed.

If something is not likely, use:

• probably won’t
• unlikely
• I doubt that

Examples:

• He probably won’t come.
• It is unlikely that they will agree.

If something is almost impossible, use:

• can’t
• impossible

Examples:

• That can’t be true.
• It is impossible to finish today.

Download Expressing Probability in English Practice Worksheet in PDF here

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Final Thoughts

Expressing probability in English helps you show how certain or uncertain you are. Instead of using only simple phrases such as I think or maybe, you can choose more precise words.

Use will, definitely and certainly when you are very sure.

Use probably and likely when something is expected to happen.

Use may, might, could and possibly when something is possible.

Use can’t, unlikely and impossible when something is not likely or not possible.

Once you learn these words and structures, your English becomes clearer, more natural and more persuasive.

And remember: very few things in life are 100% certain — except that English learners will eventually meet modal verbs again. They always come back, usually wearing a different hat.

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Related Lessons

• Modal Verbs of Deduction in English
• Modal Verbs May and Might
• Adverbs of Probability
• Modal Verbs in English
• Can, Could and Be Able To
• Sentence Transformations for B2 First
• Useful Phrases for B2 First and C1 Advanced Writing

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FAQ: Expressing Probability in English

What does expressing probability in English mean?

Expressing probability in English means showing how likely or unlikely something is. For example, She might come means it is possible, while She will definitely come means you are very sure.

What is the difference between may, might and could?

May, might and could all express possibility. May can sound slightly more formal, might is very common in everyday English, and could often suggests one possible explanation.

What is the difference between probably and possibly?

Probably means something is very likely. Possibly means something may happen, but you are less certain.

Can I say “He must to be tired”?

No. After modal verbs, do not use to. The correct sentence is: He must be tired.

What is the difference between likely and possible?

Likely means something will probably happen. Possible means something can happen, but it may not be very probable.