Direct link to YouTube video: https://youtu.be/foxkbCZpofc

Explanations + Extra Practice Activities

Modal Verbs of Deduction – The Certainty Ladder

We use modal verbs of deduction when we want to make a guess about a situation.
We are not 100% sure, but we use logic, evidence, or experience.

The Certainty Ladder

✅ 100% certain – I know (use regular verbs)

🎯 95% – MUST (very sure)

💭 90% – WILL (confident logical inference)

👍 80% – SHOULD / OUGHT TO (expected)

🤔 50% – MAY / MIGHT / COULD (maybe)

📉 20% – SHOULDN’T (expected not to)

🎯 10% – MUST NOT (strong negative deduction)

❌ 5% – CAN’T / COULDN’T (very sure it’s NOT true)

Think of it as a certainty ladder:

Higher = more sure
Lower = less sure

✅ 100% Certain – I know (Use regular verbs)

When you are completely sure, do NOT use modal verbs.
Use normal verbs in the correct tense.

Use this when:

You see it
You have a fact
There is no doubt

Examples:

I know he is at home. (I can see him.)
She is sick. (She told me.)
They finished the test already.
The shop opens at 9 a.m.
He doesn’t live here anymore.

🎯 95% Certain – MUST (Very sure)

MUST means you are almost completely sure.
You have strong evidence.

Form:

must + base verb
must + be + adjective / noun

Examples:

The lights are on. He must be at home.
She worked all night. She must be tired.
The phone is ringing. It must be important.
He has no money. He must need help.
They didn’t answer all day. They must be busy.

💭 90% Certain – WILL (Confident logical inference)

WILL is used for a very confident guess, often based on experience or logic.

Examples:

Don’t worry. He will be fine.
It’s 8:30. She will be on the bus now.
This key will open the back door.
That noise will be the neighbors.
He left early, so he will arrive soon.

👍 80% Certain – SHOULD / OUGHT TO (Expected)

SHOULD / OUGHT TO mean something is expected, but not guaranteed.

Form:

should / ought to + base verb
should / ought to + be

Examples:

The train left on time. It should arrive soon.
She studied a lot. She should pass the exam.
He lives nearby. He should be here already.
The weather report was good. It should be sunny.
This road ought to take us there faster.

🤔 50% Certain – MAY / MIGHT / COULD (Maybe)

These modals mean it’s possible, but you are not sure.

Important:
All three mean almost the same here.

Examples:

She might be at home.
He may be late today.
They could be stuck in traffic.
It might rain later.
The answer may be wrong.

📉 20% Certain – SHOULDN’T (Expected NOT to)

SHOULDN’T means something is not expected, but still possible.

Examples:

The shop shouldn’t be closed yet.
He shouldn’t be late. He left early.
This medicine shouldn’t cause problems.
She shouldn’t need help.
They shouldn’t arrive before noon.

🎯 10% Certain – MUST NOT (Strong negative deduction)

MUST NOT means you are almost sure something is NOT true because the person does not know, is not aware, or has not noticed something.

This is a logical conclusion, not a rule or order.

Use this when:

Someone has no knowledge of something
Someone shows no awareness of a fact
Important information is missing

Examples (Lack of Knowledge / Awareness)

He didn’t answer the phone. He must not know we called.
She looks surprised. She must not be aware of the change.
He keeps asking questions. He must not understand the problem.
They are talking normally. They must not know about the accident.
She posted photos online. She must not realize it’s private.

More Real-Life Examples

He brought no umbrella. He must not know it’s going to rain.
She went the wrong way. She must not be aware of the new entrance.
He’s laughing about it. He must not realize how serious it is.
They are waiting outside. They must not know the office is closed.
She deleted the email. She must not have noticed the warning.

⚠️ Important for ESL Students

MUST NOT (deduction) = I am almost sure this is NOT true
❌ It is NOT a rule or prohibition

Compare:

🚫 You must not park here. → rule
🧠 He must not know about the rule. → deduction

❌ 5% Certain – CAN’T / COULDN’T (Very sure it’s NOT true)

CAN’T / COULDN’T mean you are almost 100% sure something is impossible.

Examples:

He can’t be 15. He’s 40!
This can’t be the right answer.
She couldn’t be at work. It’s Sunday.
That can’t be his car. He sold it.
They can’t have finished already. It’s too soon.

🧠 Quick Tip for Students

Ask yourself:

How sure am I?

Then choose the modal that matches your level of certainty.

This skill will help you:

Sound more natural
Make better guesses
Speak with confidence in English

Extra Practice Activities!

Modal Verbs of Deduction – Quiz

Instructions:
Choose the BEST answer based on the situation.

Part 1: Multiple Choice

1. The lights are on, and I hear music inside.
He ___ be at home.
a) might
b) should
c) must
d) can’t

Answer: c) must

2. She studied very hard all semester.
She ___ pass the exam.
a) must
b) should
c) can’t
d) might

Answer: b) should

3. It’s only 6 a.m. The shop opens at 9.
The shop ___ be open yet.
a) must
b) may
c) shouldn’t
d) will

Answer: c) shouldn’t

4. He is smiling and laughing.
He ___ know that he is getting fired today.
a) must
b) might
c) must not
d) should

Answer: c) must not

5. It’s a cloudy day.
It ___ rain later.
a) will
b) must
c) can’t
d) might

Answer: d) might

6. She left the office at 6 p.m. as usual. It is 8 o’clock now.
She ___ be home by now.
a) could
b) should
c) can’t
d) must not

Answer: b) should

7. He says, “What meeting?”
He ___ know about it.
a) can’t
b) must not
c) will
d) should

Answer: b) must not

8. This answer is completely wrong.
It ___ be correct.
a) may not
b) must not
c) can’t
d) shouldn’t

Answer: c) can’t

9. Don’t worry. He’s very experienced.
He ___ know what to do.
a) might
b) must
c) will
d) can’t

Answer: c) will

10. I’m not sure where she is. It’s possible she’s at home.
She ___ be at home.
a) must
b) should
c) may
d) can’t

Answer: c) may

Part 2: Choose the Best Modal

Use: must / will / should / may / might / could / shouldn’t / must not / can’t

11. He did not bring an umbrella. He __________ know it’s going to rain.
Answer: must not

12. The train left on time. It __________ arrive soon.
Answer: should

13. She isn’t answering her phone. She __________ be busy.
Answer: might / may / could

14. He’s only 12 years old. He __________ be the manager.
Answer: can’t

15. The man is trying to open the door of the store, but today is Sunday. He __________ know they are closed today.
Answer: must not

Part 3: Certainty Check

Instructions:
Match each sentence to the correct level of certainty.

A = very sure
B = expected
C = possible
D = almost impossible

16. They might be late.

Answer: C

17. That can’t be true.

Answer: D

18. She should be here soon.

Answer: B

19. He must be tired.

Answer: A

Part 4: Certainty Check

A = very sure
B = expected
C = possible
D = almost impossible

20. He can’t be asleep. The lights are on and the TV is loud.

Answer: D

21. She might forget the meeting. She didn’t write it down.

Answer: C

22. The bus should arrive in five minutes. It’s usually on time.

Answer: B

23. He must be nervous. It’s his first interview.

Answer: A

24. That can’t be the right address. The numbers don’t match.

Answer: D

25. They may need more time. The task is difficult.

Answer: C

A = very sure
B = expected
C = possible
D = almost impossible

26. She should know the answer. She studied this topic.

Answer: B

27. He must be joking. Everyone is laughing.

Answer: A

28. This can’t be her bag. Her name isn’t on it.

Answer: D

29. He could miss the train. Traffic is very heavy.

Answer: C

30. The package should arrive today. It was shipped yesterday.

Answer: B

31. She must be proud. Her son just graduated.

Answer: A

Part 5: Choose the BEST Modal (Multiple Choice)

Instructions:
Choose the best answer.

32. The lights are off and no one is answering the door.
They ___ be at home.

a) might
b) should
c) can’t
d) will

Answer: c) can’t

33. She left work 30 minutes ago. The drive usually takes 20 minutes.
She ___ be home by now.

a) may
b) must not
c) should
d) can’t

Answer: c) should

34. He looks confused and keeps asking the same question.
He ___ understand the instructions.

a) can’t
b) must not
c) will
d) should

Answer: b) must not

35. It’s possible they took the wrong bus.
They ___ be on the wrong bus.

a) must
b) should
c) might
d) can’t

Answer: c) might

36. This answer is clearly wrong.
It ___ be correct.

a) may
b) should
c) must
d) can’t

Answer: d) can’t

37. He studied all night and didn’t sleep.
He ___ be exhausted.

a) might
b) must
c) shouldn’t
d) can’t

Answer: b) must