Expressing Probability: Modals and Expressions of Certainty for C1/C2 Learners

When communicating in advanced English, stating bare facts isn’t always enough. Often, you need to show how certain you are about a situation. Whether you are writing an academic essay, presenting a business forecast, or taking an advanced English exam like Cambridge C1 Advanced (CAE) or C2 Proficiency (CPE), mastering the nuances of probability is essential.

In English, we use modal verbs and specialized phrasal expressions to indicate our degree of certainty about present and future situations.

Let’s dive into the complete structural breakdown, from absolute certainty to general possibility.

1. Absolute Certainty: When You Are 100% Sure

When you have strong evidence or logical reasons to believe something is true (or untrue), you use structures of deduction.

✦ must (Positive Deduction)

We use must when reasoning or physical evidence makes a conclusion seem like the only realistic possibility.

• Formula: Subject + must + Infinitive (without 'to')
• Example: “You must be exhausted after a 12-hour flight.” (Reasoning: Long flights cause exhaustion.)
• Example: “That must be Sarah’s house; her distinct blue car is parked in the driveway.”

Nuance Check: There is a difference between fact and deduction.

• “This is her house.” (A known, verified fact)
• “This must be her house.” (A conclusion reached through observation/evidence)

✦ be bound to (Certain Predictions)

be bound to is not a modal verb, but a powerful semi-modal expression used to say that a future outcome is virtually inevitable or guaranteed based on current trends.

• Formula: Subject + be (am/is/are) + bound to + Infinitive
• Example: “With both strikers injured, the team is bound to struggle in tomorrow’s match.”
• Example: “If you don’t double-check your calculations, you are bound to make mistakes.”

✦ can't (Negative Deduction)

When you are completely certain that something is impossible based on evidence, use can't. Do not use mustn’t for this meaning.

• Formula: Subject + can't + Infinitive
• Example: “That can’t be the correct total. It’s far too high.”
• Example: “It can’t be easy managing a global corporation entirely on your own.”

2. High Probability: “If All Goes Well”

When you expect something to happen because it is logical, normal, or according to plan, you use structures of high probability.

✦ should / shouldn't

These modals imply that an outcome is highly probable if everything goes smoothly.

• Example: “Take this medication and rest; you should feel much better by Friday.”
• Example: “The traffic is light, so reaching the airport shouldn’t be a problem.”

⚠️ Warning for Advanced Writers: We do not use should to predict negative, painful, or unwanted events.

• ❌ The medical procedure should be very painful.
  • The medical procedure is likely to be very painful.

3. Medium to Low Probability: Weak Possibility

When an outcome is possible but you lack the definitive proof to be certain, you enter the realm of might, may, and could.

✦ might / may / could

These three modals share a very similar meaning of “perhaps,” but they carry different levels of formality.

• May: The most formal variant, ideal for academic and professional writing. (“There may be underlying structural issues we have yet to uncover.”)
• Might / Could: More common in spoken English and neutral writing. (“I might see you at the conference tomorrow.”)

✦ Boosting Probability with well

If you want to say that a weak possibility is actually highly likely, insert the adverb well immediately after the modal verb.

• Formula: Subject + might/may/could + well + Infinitive
• Example: “The weather forecast is terrible; the outdoor event may well be cancelled.”
• Example: “Given his track record, that rumor could well be true.”

✦ Negative Weak Possibility

To express a weak negative possibility, use may not or might not (or mightn’t).

• Example: “We may not need to hire an external consultant, but let’s keep our options open.”
• Example: “James might not attend the symposium today due to a prior engagement.”

Critical Distinction: couldn't behaves differently than may not/might not. While might not means “perhaps it won’t happen,” couldn’t means “it is absolutely impossible.”

4. General vs. Specific Possibility: can vs. could

A common pitfall for advanced learners is using can for specific predictions.

• Specific Situations: Never use can to talk about the probability of a specific event. Instead, use may, might, or could.
  • ❌ Alex can be in the garden right now.
    • Alex could/might/may be in the garden right now.
• General Situations: Use can exclusively to state a general truth—something that is known to happen from time to time or is theoretically possible.
  • • “Noisy neighbors can be incredibly frustrating when you live in a flat.” (Meaning: It happens generally.)
    • “The temperature can drop below freezing here during January.”

The Probability Scale: Quick Reference Guide

Structure	Level of Certainty	Example
must / be bound to	100% Certain (Positive)	“They are bound to win the league.”
can’t / couldn’t	100% Certain (Negative)	“That can’t be the right answer.”
should / shouldn’t	80% Expected (If all goes well)	“The package should arrive tomorrow.”
may well / might well	70% Highly Likely	“She may well decline the promotion.”
may / might / could	40-50% Possible	“It could rain later this afternoon.”
may not / might not	40-50% Possibly Not	“They might not accept our terms.”
can	General Theoretical Possibility	“Winters here can be brutally harsh.”

Interactive Grammar Lab: Modals of Probability

Test your mastery of advanced modals and expressions of probability with our interactive exercise engine below. Choose the correct modal based on the context clues and nuances provided!