B1Modal Verbs

Modal Verbs: Probability

What is it?

Modal verbs of probability allow us to express how certain we are about something. We use "must" for near certainty, "may/might/could" for possibility, and "can't/couldn't" for near impossibility. For the present, we use modal + infinitive; for the past, we use modal + have + past participle.

How to form it

Subject	Positive	Negative	Question
Present deduction (certain)	She must be at home — the lights are on.	He can't be the manager — he's only 22.	Could that be John over there?
Present possibility	It might/may/could rain later.	He might not come — he's very busy.	Might she be in a meeting?
Past deduction	They must have left already.	She can't have made that mistake.	Could he have forgotten?

• Degrees: must (almost certain) > should (likely) > may/might/could (possible) > can't/couldn't (almost impossible)
• For present probability: modal + base infinitive.
• For past probability: modal + have + past participle.
• "Must" for probability is different from "must" for obligation.
• In questions, "could" is more natural than "must" or "might".

When to use it

1
"Must" = near certain deduction (positive)
She's been working for 12 hours. She must be exhausted.
2
"Can't / couldn't" = near certain deduction (negative)
He just ate a huge meal. He can't be hungry already.
3
"May / might / could" = possible but uncertain
I'm not sure. She might be in a meeting.
4
"Should" = expected or likely
They left two hours ago. They should be there by now.
5
Past probability: modal + have + past participle
She hasn't replied. She might have missed the message.

Common mistakes

She must have be very tired.

She must have been very tired.

For past probability: modal + have + past participle ("been", not "be").

He can't have forgotten — he has a good memory.

He can't have forgotten — he has a good memory. ✓ (this is correct)

This sentence is actually correct. "Can't have" is used for past impossibility.

It might rains later.

It might rain later.

After modal verbs, always use the bare infinitive — no -s or -ed.

Quick reference

"Must" = almost certain (positive deduction).
"Can't / couldn't" = almost certain (negative deduction).
"May / might / could" = possible (about 50% or less).
"Should" = expected or likely.
Past probability: modal + have + past participle.
The strength of certainty: must > should > might/may/could > can't.

Natural conversation example

Two colleagues are wondering where their manager is.

Anna

Has anyone seen David today? He's not in his office.

Rob

He might be in the boardroom — there was a big meeting scheduled.

Anna

His coat is here, so he can't have gone home.

Rob

True. He must be somewhere in the building then.

Anna

Could he have gone for lunch early?

Rob

He might have. It's nearly 12:30.

Anna

He should be back soon, then. He never takes long lunch breaks.

Rob

I'll send him a message just in case.

Practice Exercises

Fill in the blank using must, might, can't, or could.

1.
She's been awake for 24 hours. She __ be exhausted. (near certain)
2.
He just had dinner. He __ be hungry already. (near impossible)
3.
I'm not sure where she is. She __ be in a meeting. (possible)
4.
The lights are off. They __ have gone to bed. (past deduction)
5.
She __ have taken the wrong bus — she knows the route well. (past impossibility)
6.
He __ come to the party. He said he was busy but not sure. (possibility)
7.
They left three hours ago. They __ be there by now. (expectation)
8.
That __ be right — the numbers don't add up. (impossibility)
9.
She __ have forgotten — she always forgets everything. (possible past)
10.
He won every award this year. He __ be incredibly talented. (certain)

Modal Verbs of Obligation All Topics Negative Modals

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