Modal Verbs of Probability: how to express possibility and certainty in English

Have you ever guessed that someone would definitely be late, predicted the weather thanks to aching knees, or figured out who was calling even before picking up the phone? All of these are assumptions we make every day. In English, there are special modal verbs that help express the likelihood of events with varying degrees of certainty — modal verbs of probability. Let's figure out how to use them correctly to speak English more accurately.

Contents

1. What are modal verbs of probability
2. Modal verbs categorized by levels of probability
3. How the choice of a modal verb influences information perception
4. How to use modals of probability
4.1. Expressing probability in the past
5. Practice exercise
6. FAQs

What are modal verbs of probability

Modals of probability are used to express assumptions about events, situations, or facts in the present, past, and future. They help convey different levels of certainty or uncertainty in statements.

These modal verbs are commonly used in both spoken and written English, particularly in making predictions, assumptions, and logical conclusions. For example:

She must be at work now.
He can't be the thief.
They might come later.

Modal verbs categorized by levels of probability

Modal verbs of probability range from complete certainty to mere assumption or even doubt. For example, the verb “must” indicates a high degree of certainty, while “might” or “could” suggest possibility without confidence, and “can’t” expresses absolute impossibility. Let's explore the main types and their usage in speech:

The degree of possibility	Modal verbs	Examples
High probability (90-100%)	must	He must be tired. This must be a mistake.
Medium probability (50-70%)	may	It may rain tomorrow.
Low probability ( ≤ 50%)	might / could	He could win the lottery. They might not come to the party.
No probability	can't	This can't be true! He can't know the answer.

How the choice of a modal verb influences information perception

Modal verbs play a crucial role in communication, as they not only convey the meaning of possibility, necessity, or probability but also affect how a statement is perceived. Even a slight change in a modal verb can make a phrase sound more confident, neutral, or uncertain.

For example, “must” expresses strong certainty, while “might” only suggests a possibility. That’s why choosing the right modal verb helps to convey the intended shade of meaning and avoid misunderstandings in communication.

He must be at work.	He might be at work.	He can't be at work.
They must have forgotten about the meeting.	They may have forgotten about the meeting.	They can't have forgotten about the meeting.
It must be a joke.	It could be a joke.	It can't be a joke.

Here, we see how “must” expresses high certainty, “might/may” indicate a small probability, and “can't” completely denies the possibility.

How to use

Modal verbs are “independent” and do not require endings in affirmative sentences or auxiliary verbs to form negative sentences and questions.

Affirmative sentences

Modal verbs do not change according to the subject:

✗ She ~~musts~~ be tired.
✓ She must be tired.

The modal verb is followed by the infinitive without to:

✗ He ~~must to go~~ home.
✓ He must go home.

✗ They ~~can't to be~~ late.
✓ They can't be late.

Negative sentences

Modal verbs do not require auxiliary verbs to form a negative sentence. Simply add “not” after the modal verb:

✓ She can't be at home.
✓ They might not come to the party.

Interrogative sentences

A modal verb is placed at the beginning of a question without any auxiliary verbs (no “do”, “does”, or “did”):

✓ Can he be the new teacher?
✓ Must we finish the project today?
✓ Might she be sleeping now?

Expressing probability in the past

To express the probability of an action in the past, add the auxiliary verb “have” followed by the past participle (the third form of the verb):

Modal V + have + V3/ed

Must have	Certainty that something happened
Can't have / couldn't have	Certainty that something could not have happened
May have / might have / could have	Possibility that something happened

She must have forgotten about the meeting.
He might have gone home.

Negative forms

To suggest that an action did not happen, simply add “not” after the modal verb:

She can't have forgotten about the meeting.
He might not have seen your message.

Practice exercise

Now you will be able to better understand and use modals of probability in English. Practice with the exercise below to reinforce your knowledge.

Choose the correct modal verb.

You can learn more about modal verbs in the following articles:

FAQs

1. Which modal verb expresses the strongest certainty?

“Must” expresses almost 100% certainty about the correctness of a guess, but only in affirmative sentences. For example:

He must be the manager.

2. How to say something is impossible in English?

In terms of probability, we cannot use “must” in negotiations. Instead, use “can't” in negative sentences to express impossibility:

This can't be true.

3. Is there a difference between “may”, “might”, and “could”?

These words are similar in meaning, but “might” and “could” generally express a lower probability than “may”. They often give the statement a sense of uncertainty and hypothetical nature. Let’s compare:

It may rain tomorrow.
It might rain tomorrow.
It could rain tomorrow.