Guide to Modals of Certainty and Uncertainty – TOEIC® Preparation

In English, several linguistic tools allow us to qualify probability, express certainty, or conversely, uncertainty regarding an event. Modal verbs (accompanied by certain equivalent phrases) constitute essential markers of this gradation: they indicate the degree of conviction one has concerning the realization of a fact, whether it is almost certain, likely, conceivable, or conversely, very improbable.

This course presents the main modals and structures used to express different levels of probability and uncertainty.

1. “Must” to Indicate Near-Absolute Conviction

The modal verb “must” is used to convey a very strong conviction: one is practically certain that an event is real or will happen.

Caution: Do not confuse the meaning of “must” related to probability with the one expressing obligation (“You must complete this task” = “Tu dois accomplir cette tâche”).

She must be exhausted after such a long day.(She must be exhausted after such a long day.)
They must have already departed.(They must have already left.)
You must be kidding!(You must be joking!)

To delve deeper into obligation modals, consult our dedicated course on modals expressing obligation.

“Must have + Past Participle” to Express Strong Past Conviction

The structure “must have + Past Participle” is used to express near-certainty regarding a past event.

She must have misplaced her phone somewhere.(She must have misplaced her phone somewhere, we are almost sure.)
They must have departed hours ago.(They must have left hours ago.)

2. “Can’t” to Signify Impossibility

The modal verb “cannot” (or its contraction “can’t”) is used to express near-certainty that a statement is false (an impossibility). In other words, one is practically sure that a situation is not true or cannot happen.

They can’t be telling the truth!(It's impossible that they are telling the truth!)
She can’t have the information; we only just discovered it.(She cannot have the information; we have only just discovered it.)
He cannot be home if his vehicle isn't parked outside.(He cannot be home if his vehicle isn't parked outside.)

“Can’t have + Past Participle” to Signify Past Impossibility

The form “can’t have + Past Participle” conveys the near-certainty that an event did not happen or was impossible in the past.

She can’t have been aware of the surprise gathering.(It is impossible that she was aware of the surprise gathering.)
They cannot have noticed us; we were completely concealed.(They cannot have noticed us; we were completely concealed.)

3. “Should” to Translate High Probability

The modal verb “should” expresses high probability. It suggests that it is very likely an event will occur; it is a kind of logical prediction.

“Should” can also mean advice (“You should consult a specialist”), but in the context of probability, it refers to the idea that “logically, this should happen.”

He should be here any minute now.(He should be here any minute now.)
It should be clear skies tomorrow based on the forecast.(It should be clear skies tomorrow based on the forecast.)
You should succeed in the test if you prepare thoroughly.(You should succeed in the test if you prepare thoroughly.)

“Should have + Past Participle” to Express Logical Past Expectation

The structure “should have + Past Participle” conveys high probability or a logical expectation concerning the past, often accompanied by a nuance of reproach or regret.

It should be noted that “should have + Past Participle” can also convey reproach or regret, beyond the simple expression of probability.

He should have been here by 10am this morning.(He should have been here by 10 am this morning.)
We should have reserved our seats in advance.(We should have reserved our seats in advance.)

4. “Be bound to” to Indicate Inevitable Event

The expression “be bound to” conveys near-certainty. It is used to refer to an event that will necessarily happen, often inevitably.

She is bound to achieve her goals with such dedication.(She is bound to achieve her goals with such dedication.)
They are bound to triumph if they maintain this level of performance.(They are bound to triumph if they maintain this level of performance.)
This regulation is bound to impact numerous companies.(This regulation is bound to impact numerous companies.)

“Was / were bound to” to Designate an Inevitable Past Event

Generally, one avoids the form “was bound to have…” to express that an event was inevitable in the past; one prefers the simple past “was bound to…”.

She was bound to succeed given her level of preparation.(She was bound to succeed given her level of preparation.)

5. “Be likely to” to Translate Significant Probability

The structure “be likely to” expresses high probability, meaning there is a strong chance that an event will materialize.

They are likely to be delayed due to the roadworks.(They are likely to be delayed due to the roadworks.)
She is likely to receive a promotion in the near future.(She is likely to receive a promotion soon.)
This item is likely to be popular with customers.(This item is likely to be popular with customers.)

“Was / were likely to” to Translate Past Probability

Generally, we avoid the form “was/were likely to have left…” to express that an event was probable in the past; one prefers the simple past “was/were likely to…”.

They were likely to depart before the bad weather arrived.(They were likely to depart before the bad weather arrived.)

6. “May” to Express Moderate Possibility

The modal verb “may” is used to indicate that something is conceivable, without being totally certain. We are looking at a medium to high probability.

She may attend the meeting.(She may attend the meeting.)
We may visit Italy next autumn.(We may visit Italy next autumn.)
It may seem straightforward, but it's actually quite complex.(It may seem straightforward, but it's actually quite complex.)

“May have + Past Participle” to Express Past Possibility

The structure “may have + Past Participle” expresses a possibility or probability concerning a past event, without total certainty.

He may have missed his connection.(He may have missed his connection.)
They may have overlooked calling you back.(They may have overlooked calling you back.)

7. “Could” to Signify General Possibility

The modal verb “could” allows for the expression of a possibility, often slightly weaker than may, or a theoretical hypothesis.

It could be raining this afternoon.(It could be raining this afternoon.)
You could discover a better opportunity if you persist.(You could discover a better opportunity if you persist.)
She could be the ideal candidate for this role, though I'm not completely convinced.(She could be the ideal candidate for this role, though I'm not completely convinced.)

“Could have + Past Participle” to Express a Past Possibility

The form “could have + Past Participle” expresses a possibility (general or hypothetical) in the past. This possibility remains less certain than if one uses “may have”.

He could have missed his departure.(He could have missed his departure.)
They could have forgotten to contact you.(They could have forgotten to contact you.)

8. “Might” to Translate Low Possibility

The modal verb “might” is used to express a hypothetical possibility, or one less certain than with may or could.

He might travel to Berlin.(He might travel to Berlin.)
She might contact you this evening, but don't rely on it.(She might contact you this evening, but don't count on it.)
They might be at the theater, though they didn't confirm.(They might be at the theater, although they didn't confirm.)

“Might have + Past Participle” to Translate Low Past Possibility

The structure “might have + PP” allows for the expression of a low or uncertain possibility in the past.

He might have departed earlier, I'm not certain.(He might have departed earlier, I'm not certain.)
They might have noticed us, though I have my doubts.(They might have noticed us, although I have my doubts.)

Conclusion

To excel at the TOEIC®, it is crucial to master these different modals and expressions that allow for qualifying probability and uncertainty. Each modal conveys a specific nuance: from near-certainty (must) to marked uncertainty (might). Integrating this gradation will help you better interpret the statements you read or hear, and express yourself with precision both in writing and speaking.

As with every module dedicated to modals, you will find below a summary table as well as the essential elements to memorize and the pitfalls to avoid.

Summary Table: Modals of Probability and Uncertainty

Modal / Expression	Level of Certainty	Meaning	Example
Must	Near certainty (strong affirmation)	We are practically sure that it is true.	She must be exhausted after such a long day.
Can't / Cannot	Near certainty that it is false (impossibility)	We are practically sure that it is not true or not possible.	They can't be telling the truth!
Should	High probability	It is likely or logical that an event will happen.	He should be here any minute now.
Be bound to	Near certainty (inevitable)	Event considered inevitable.	She is bound to achieve her goals with such dedication.
Be likely to	High probability	There is a strong chance that the action will materialize.	They are likely to be delayed due to the roadworks.
May	Medium/High probability	Real possibility, without total certainty.	She may attend the meeting.
Could	General possibility	Sometimes a less certain possibility than may, often hypothetical.	It could be raining this afternoon.
Might	Weaker possibility	More hypothetical or uncertain than may or could.	He might travel to Berlin.

Essential Points to Remember About Probability Modals

Certainty Scale
- Must (practically sure it is true)
- Can’t / Cannot (practically sure it is false)
- Should, be bound to, be likely to (high probability)
- May, Could (medium to more moderate probability)
- Might (lower possibility)
Modal Selection According to Context
- Consider the register and phrasing: should and be likely to are often used in a neutral or slightly formal register.
- Must and can’t are very categorical in terms of certainty or impossibility (they can sometimes seem abrupt or overly assertive depending on the situation).
- May, might, and could leave room for uncertainty and are useful for discussing plans, hypotheses, or future events that cannot be confirmed.
Caution with the negative form of must: Must not (mustn’t) does not always convey the same meaning as can’t.
- Mustn’t is often perceived as “prohibition” (negative obligation), whereas can’t can indicate impossibility.
- In the context of probability, it is rather can’t that is preferred to express that one is almost certain that a statement is false.
Past Tense Structures
- Use the construction modal + have + past participle to express a probability or impossibility concerning a past event.
  - She must have arrived late (She must have arrived late)
- Be careful not to overload a sentence with complex past structures (e.g., was likely to have done can be simplified).

Other Resources on Modals

Here are our other modules on modals you can consult to optimize your TOEIC® preparation:

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