Published at February 17, 2025

Course on Expressing Probability - TOEIC® Preparation

TOP-Students™

In English, there are several ways to express probability, certainty, or uncertainty. Modals (and some related expressions) play a key role in this nuance: they allow you to say if an event is almost certain, probable, possible, or, conversely, very unlikely.

In this course, we will review the main modals and expressions used to express probability or uncertainty.

1. "Must" to express near-certainty

We use "must" to talk about a near-certainty, we are almost sure that something is true or will happen.

Be careful not to confuse the meaning of "must" (probability) with "must" which expresses obligation ("You must do your homework" = "You have to do your homework").

He must be tired after working so late.
They must have left already.
You must be joking!

For more information on obligation modals, you can read our course on obligation modals.

"Must have + Past Participle" to express near-certainty in the past

We use the form "must have + Past Participle" to express the near-certainty that an action took place in the past.

He must have forgotten his keys at home.
They must have left already.

2. "Can't" to express impossibility

We use "cannot" (or its contracted form "can't") to express a near-certainty that something is false (an impossibility). Basically, we are almost sure that something is not true or not possible.

They can't be serious!
She can't know the answer; we just found out ourselves.
He cannot be at home if his car is not there.

"Can't have + Past Participle" for impossibility in the past

We use the form "can't have + Past Participle" to express the near-certainty that something did not happen or was not possible in the past.

She can't have known about the surprise party.
They cannot have seen us; we were hidden.

3. "Should" to express strong probability

We use "should" to express a strong probability. Basically, we estimate that it is very likely that an action will occur, it's a kind of logical prognosis.

"Should" can also express advice ("You should see a doctor"), but in the context of probability, it refers to the idea that "logically, it should happen".

She should arrive soon.
It should be sunny tomorrow according to the weather forecast.
You should pass the exam if you study hard.

"Should have + Past Participle" to express strong probability in the past

We use "should have + Past Participle" to express a strong probability or a logical expectation in the past, often implying reproach or regret.

It should be noted that "should have + Past Participle" can also express a reproach or regret, beyond simple probability.

He should have arrived by 11am this morning.
We should have booked our tickets earlier.

4. "Be bound to" to express near-certainty, something inevitable

We use the expression "be bound to" to express a near-certainty. We use it to talk about an event that will necessarily happen, often inevitably.

He is bound to succeed with all that preparation.
They are bound to win if they keep playing this well.
This new law is bound to affect many businesses.

"was / were bound to" to say something was inevitable in the past

In general, we avoid the form "was bound to have…" to express that something was inevitable in the past, we prefer a simple past "was bound to…".

He was bound to succeed with all that preparation.

5. "Be likely to" to express high probability

We use "be likely to" to express a high probability, that is, a strong chance that an action will occur.

They are likely to arrive late because of the traffic.
She is likely to get a promotion soon.
That product is likely to sell well.

"was / were likely to" to say something was probable in the past

In general, we avoid the form "was/were likely to have left…" to express that something was probable in the past, we prefer a simple past "was/were likely to…".

They were likely to leave before the storm hit.

6. "May" to express medium probability

We use "may" to say that something is possible, but not 100%. We are on a medium to high probability

He may come to the party.
We may travel to Spain next summer.
It may look easy, but it's actually quite complicated.

"May have + Past Participle" to express medium probability in the past

We use "may have + Past Participle" to express a possibility or a probability regarding a past event, without absolute certainty.

He may have missed his flight.
They may have forgotten to call you.

7. "Could" to express general possibility

The modal "could" allows expressing a possibility, often less strong than may, or a theoretical hypothesis.

It could rain later.
You could find a better job if you keep looking.
He could be the right person for the job, but I'm not entirely sure.

"Could have + Past Participle" to express possibility in the past

We use "could have + Past Participle" to express a possibility (general or hypothetical) in the past. This possibility remains less certain than if we use "may have".

He may have missed his flight.
They may have forgotten to call you.

8. "Might" to express a weak possibility

The modal "might" can be used to express a hypothetical possibility, or less certain than if we use may or could.

He might go to London.
She might call you later, but don't count on it.
They might be at the cinema, but they didn't say.

"Might have + Past Participle" to express a weak possibility in the past

We can use "might have + PP" to express a weak or uncertain possibility in the past.

He might have left already, I'm not sure.
They might have seen us, but I doubt it.

Conclusion

To succeed in the TOEIC®, it is very important to master these different modals and expressions that allow you to nuance probability and uncertainty. Each modal brings a specific nuance: from very certain (must) to very uncertain (might). Remembering this gradation will help you better understand the sentences you read or hear, and better express yourself in writing and speaking.

As with each chapter on modals, you will find a summary table as well as the key points to remember and pay attention to below.

Summary on modals for expressing probability or uncertainty

Modal / Expression	Degree of probability	Meaning / Nuance	Example
Must	Near-certainty (strong affirmation)	We are almost sure it's true.	He must be tired after working so late.
Can't / Cannot	Near-certainty that it's false (impossible)	We are almost sure it's not true or not possible.	They can't be serious!
Should	Strong probability	Probable or logical that something will happen.	She should arrive soon.
Be bound to	Near-certainty (inevitable)	Event considered inevitable.	He is bound to succeed with all that preparation.
Be likely to	High probability	We consider there is a strong chance the action will occur.	They are likely to arrive late because of the traffic.
May	Medium/high probability	Real possibility, without absolute certainty.	He may come to the party.
Could	General possibility	Possibility sometimes less certain than may, often hypothetical.	It could rain later.
Might	Weaker possibility	More hypothetical or uncertain than may or could.	He might go to London.

Key points to remember about modals for expressing probability or uncertainty

Gradation of certainty
- Must (almost sure it's true)
- Can't / Cannot (almost sure it's false)
- Should, be bound to, be likely to (strong probability)
- May, Could (medium to weaker probability)
- Might (weaker possibility)
Choice of modal according to context
- Consider the tone and formulation: should and be likely to are often used in a slightly more formal or neutral register.
- Must and can't are very strong in terms of certainty or impossibility (they can sometimes seem abrupt or too assertive in certain situations).
- May, might, and could leave room for uncertainty and are practical for talking about plans, hypotheses, or future events that cannot be confirmed.
Attention to the negative form of must : Must not (mustn't) does not always mean the same as can't.
- Mustn't is often perceived as "not allowed" (negative obligation), whereas can't can indicate impossibility.
- In terms of probability, it is rather can't that we use to say we are almost certain that something is false.
Past forms
- Use the structure modal + have + past participle to express a probability or an impossibility regarding a past action.
  - He must have arrived late (He must have arrived late)
- Be careful not to overload a sentence with heavy past modals (was likely to have done, for example, can be simplified).

Other courses on modals

Here are our other courses on modals that you can consult to prepare for the TOEIC®: