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Teacher cue
B1 · English Grammar

Modals of
Deduction

This story can't be true — the details don't seem right.

modal + base verb how sure are you?
The idea

Guessing from evidence

When you're not stating a fact but making a logical guess, use a modal of deduction. The verb after it is always the base infinitive.

The structure
modal (+ not) + base infinitive

The news must be fake.
(never "must to be" / "must is")

The idea
evidence → conclusion

She isn't answering. She must be in a meeting.
(a guess, not a fact)

Rule 1

must — you're sure it's true

Use must when the evidence makes you about 99% certain something is true.

You've been working since 6 a.m. — you must be exhausted.
The lights are on. They must be back from their trip.
They wrote the book themselves, so they must know the answer.
Rule 2

might / may / could — you're not sure

Use these three when something is possible but you can't be certain. They mean the same here.

This story might be true, but it may not be 100% accurate.
The article has no sources — it could be fake news.
I'm not sure where Ana is. She may be at the gym.
Rule 3

can't — you're sure it's NOT true

The opposite of must in deduction is can't — never mustn't.

almost certain: yes → The news must be fake.
not sure (maybe) → That story might / may / could be fake.
almost certain: no → This story can't be true.
⚠ Watch out

Common mistakes

The news must to be fake.

Correct: The news must be fake.

He mustn't be at home — the lights are off.

Correct: He can't be at home.

She must is very tired.

Correct: She must be very tired.

That story can be true — it's impossible!

Correct: That story can't be true.

Now it's your turn

Let's
practice.

Say the answer out loud — then tap to check.

Practice 1
tap the blank to reveal

Fill the gap

You're almost certain the news is fake. → The news(certain: yes)be fake.
You aren't sure. → Do you think that story(maybe)be fake?
You're almost certain the news is true. → The news(certain: no)be fake.
He's been traveling for 20 hours. He(certain: yes)be exhausted.
I hear a noise upstairs. It(maybe)be the cat.
Practice 2
tap a sentence to reveal the error

Spot the mistake

Her coat is stillA on the hook, soB she can't to beC outside inD this weather.

Cshe can't to be → she can't be

✓ modals take the bare infinitive

Everybody in the offices knowA the code, soB the thief mustC have beenD someone from outside.

AEverybody in the offices know → Everybody in the offices knows

✓ everybody is singular → knows

She can'tA be at work becauseB her car isC still here, so she must to beD at home.

Dso she must to be → so she must be

✓ no to after must

Practice 3
tap to reveal a model answer

Complete the sentence

My neighbor's lights have been off all week. He
That price is far too good. It
Someone's knocking at the door. It
She got 100% on the hardest exam. She
Speak & write

Your turn

Use must, might / may / could and can't.

Your friend hasn't replied all day. What are two possible explanations?
You read a shocking headline online. How sure are you it's true? Why?
Look at a classmate's bag or desk. Make two deductions about their day.
You hear a strange noise at night. What can't it be? What might it be?
Recap

Remember this

must
~99% sure it's true
might / may / could
maybe — you're not sure
can't
~99% sure it's NOT true
after the modal
base infinitive — never "to"

Sure it's true? must. Sure it's not? can't. Not sure? might.

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