Modals of Possibility in English

Overview

Modal verbs of possibility -- may, might, could, must, and can't -- are used to express how certain or uncertain you are about something. They range from near-certainty ("She must be tired") to impossibility ("He can't be 50 years old"). Mastering these modals allows you to express nuance, speculation, and deduction in English.

At the CEFR B1 level, you are expected to go beyond basic obligation modals (should, must) and learn to use modals for speculation about the present and future. This is a significant step in your English development because it allows you to discuss probability, make guesses, and draw conclusions -- skills that are essential in both casual and professional conversation.

Understanding the scale of certainty is the key to this topic. English has a clear hierarchy: must (very sure) > may/could/might (possible) > can't/couldn't (impossible). Getting the right modal for the right level of certainty makes your English sound precise and natural.

How It Works

The Scale of Certainty

Modal	Certainty level	Meaning	Example
must	~95% sure	I'm almost certain this is true	She must be at home. (Her car is here.)
may	~50%	It's possible	He may come to the party.
could	~50%	It's possible	That could be true.
might	~30-50%	It's possible (slightly less sure)	It might rain later.
can't / couldn't	~0%	I'm almost certain this is NOT true	He can't be 60. (He looks 40.)

Affirmative, Negative, and Question Forms

Form	Structure	Example
Affirmative	modal + base verb	She might come.
Negative	modal + not + base verb	It might not work.
Question	Modal + subject + base verb?	Could it be true?

Must vs. Can't (Logical Deduction)

Must and can't are used when you are making a logical deduction based on evidence:

• "He's been running for an hour. He must be tired." (logical conclusion)
• "She just ate a huge meal. She can't be hungry." (logical conclusion)

This is different from must for obligation ("You must wear a seatbelt"), which you learned at A2.

Examples in Context

English	Note
She might come tomorrow.	Uncertain possibility
It could be true.	Possible, not certain
He must be tired -- he's been working all day.	Strong deduction based on evidence
She can't be 50 -- she looks so young!	Impossibility based on evidence
It may rain this afternoon.	Moderate possibility
They could be at the restaurant already.	Speculating about current location
You might not like it.	Possible negative outcome
That can't be right.	Rejecting something as impossible
It must be expensive. (Look at this place!)	Deduction from what you see
She may not know about the meeting.	Possible that she is unaware
Could this be a mistake?	Question expressing doubt
He might be stuck in traffic.	Offering a possible explanation

Common Mistakes

Using "must" for possibility instead of deduction

• Wrong: It must rain tomorrow. (meaning: maybe it will rain)
• Right: It might/could rain tomorrow.
• Why: Must for deduction means you are almost certain, based on evidence. For simple possibility, use might, may, or could.

Confusing "must not" and "can't" for deduction

• Wrong: He mustn't be at home. (meaning: I'm sure he isn't)
• Right: He can't be at home. (deduction: his car is gone)
• Why: For negative deduction (you're sure something is NOT true), use can't or couldn't. Mustn't means prohibition ("You mustn't smoke here"), not negative deduction.

Using "can" instead of "could/might/may" for possibility

• Wrong: She can be at the office.
• Right: She could/might/may be at the office.
• Why: Can is used for general ability or permission, not for speculating about specific situations. Use could, might, or may for possibility.

Adding "to" after possibility modals

• Wrong: She might to come tomorrow.
• Right: She might come tomorrow.
• Why: Modal verbs are followed directly by the base form of the verb, without to.

Usage Notes

There is very little difference between might, may, and could for possibility. May is slightly more formal than might, but in everyday conversation they are interchangeable. Could also works in most contexts.

Must for deduction is much more common in British English. American English speakers often prefer "has to" or "has got to" for the same meaning: "He's got to be tired" (American) vs. "He must be tired" (British/universal).

In questions, could is the most natural choice for possibility: "Could it be a mistake?" Might in questions is possible but less common ("Might she be wrong?"). May in questions for possibility sounds very formal and is rare in modern English.

When you want to soften a statement or avoid being too direct, possibility modals are very useful: "You might want to check that again" is much softer than "Check that again."

Practice Tips

• Evidence and deduction: Look at photos or situations and practice making deductions: "There are lots of cars -- it must be a popular restaurant." "The lights are off -- they can't be home."
• Certainty scale: For the same situation, write three sentences at different certainty levels: "She must be a teacher" / "She might be a teacher" / "She can't be a teacher." Explain what evidence would lead to each conclusion.
• News speculation: Read news headlines and speculate about details using possibility modals: "The company might be in financial trouble." "This could mean prices will rise."

Related Concepts

• Prerequisite: Should and Must -- understanding basic modal verbs for obligation and advice prepares you for modals of possibility
• Next steps: Modal Verbs - Past Deduction -- extends possibility modals to talk about deductions about past events
• Next steps: Hedging Language -- uses possibility modals as part of a broader softening strategy