Explanation
Exercises
Modal verbs of deduction — must / can’t / may / might / could

What they mean (probability)

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  • can’t = (almost) impossible / I’m sure it’s NOT true.
  • may / might / could = maybe true (uncertain possibility).
  • must = (almost) certain / I’m sure it’s true.

All these modals express a guess based on evidence.

Forms you’ll use

TimeStructureExample
Now / futuremodal + base verbHe must be tired. • She might know the answer.
Action in progressmodal + be + -ingThey must be having lunch now.
Past eventmodal + have + V3He must have left. • They might have missed the bus. • It can’t have rained.
Past in progressmodal + have been + -ingShe might have been waiting for hours.

must (sure/almost sure)

  • Strong positive deduction: The lights are off — they must be asleep.
  • Past: The door was locked — he must have left earlier.

can’t (sure/almost sure NOT)

  • Present: She can’t be at home; I saw her at work.
  • Past: He can’t/couldn’t have taken it — he was abroad.

⚠️ Don’t use mustn’t for deduction. Mustn’t = prohibition.

may / might / could (possible)

  • Present: That noise might be the neighbours.
  • Past: She may/might/could have forgotten the time.

In deduction, could ≈ may/might (often slightly weaker). Negative: may/might not (not *can not).

Don’t use can for a specific deduction

  • ❌ He can be at home now.
  • ✅ He may/might/could be at home now.

Can is fine for general possibility: Winters can be very cold here.

Quick examples

  • Evidence: Keys aren’t here. → They must be in the car.
  • Evidence: It’s silent. → They might be studying.
  • Past: Phone was off. → She could have been sleeping.
  • Past (not true): Ticket unused. → He can’t have travelled.
Exercises:
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Modal verbs of deduction: Must, may, might, could, can’t

Choose all the correct modal verbs of deduction for each gap below. In some sentences there are TWO possible correct answers.

Modal verbs of deduction: Must, may, might, could, can’t

Choose the correct modal verbs of deduction to complete the sentences below.
1
The phone is ringing. It ____ be the courier — the tracking says “Out for delivery.”
A.
B.
C.
D.
E.
2
She ____ be in love with him — yesterday she told me she hates him.
A.
B.
C.
D.
E.
3
He ____ be at the gym right now — he often goes at this hour. (More than one answer possible)
A.
B.
C.
D.
E.
4
This is not possible — you ____ be serious!
A.
B.
C.
D.
E.
5
If he drives a brand-new Jaguar, he ____ be quite rich.
A.
B.
C.
D.
E.
6
You should pick up the phone — it ____ be an important call. (More than one answer possible)
A.
B.
C.
D.
E.
7
He ____ know the answer — he helped write the textbook.
A.
B.
C.
D.
E.
8
You ____ be right, but it’s better if we check. (More than one answer possible)
A.
B.
C.
D.
E.
9
He ____ be in class — I just saw him outside the library.
A.
B.
C.
D.
E.
10
A: Who’s at the door? B: I’m not sure — it ____ be John. (More than one answer possible)
A.
B.
C.
D.
E.

Modal verbs of deduction: Must, may, might, could, can’t

Complete the dialogues with must, might, might not or can’t.

Dialogue 1

1
A: There’s a laptop on the desk with our logo.
B: It be Jason’s — he’s the only one with that sticker.
2
A: Wait, Jason is in Berlin this week.
B: It be his.
3
A: Maybe it’s Mia’s?
B: It be — she borrowed one from IT.
4
A: But Mia uses a Mac. This is a PC.
B: Then it be hers.
5
A: The login screen says “Guest”.
B: It be a spare from IT.

Dialogue 2

6
A: Ben is thirty minutes late. He ran a marathon yesterday; he
be exhausted.
7
A: Or the traffic is heavy;
he be stuck.
8
B: He just posted a photo from the gym — he
be exhausted.
9
A: If the train is delayed, he
arrive by eight.
10
B: He just texted “parking the car” — he
be close.