Modal verbs of deduction help us make logical guesses about the present or the past. When we say She must be tired, He might have forgotten, or That can’t be true, we are using evidence to show how certain we are. In this lesson, you will learn how to use must, might, may, could and can’t for deduction, with clear examples, common mistakes and practice exercises.

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What are Modal Verbs of Deduction?

Deduction means using evidence or information to decide what is probably true.

Imagine this situation:

You see your neighbour carrying three heavy suitcases to his car.

You might say:

• He must be going on holiday.
• He might be moving house.
• He can’t be going to work with all those suitcases.

You do not know the truth for certain. However, you are using the evidence to make a logical guess.

The main modal verbs of deduction are:

must	I am almost sure this is true	very certain
may	perhaps this is true	possible
might	perhaps this is true	possible
could	perhaps this is true	possible
can’t	I am almost sure this is not true	very certain

Modal Verbs of Deduction in the Present

When we make deductions about the present, we usually use this structure:

modal verb + infinitive without to

Examples:

• She must be tired.
• He might know the answer.
• They could be at home.
• That can’t be true.

Must for Deduction

We use must when we are almost certain that something is true.

We use it when the evidence strongly suggests that our conclusion is correct.

• She has been working all day. She must be exhausted.
• The kitchen smells wonderful. Someone must be cooking dinner.

Can’t for Deduction

We use can’t when we are almost certain that something is not true or is impossible.

• That can’t be John. John is much taller.
• She can’t be at home. Her car isn’t outside.

Can’t is the opposite of must in deduction.

Compare:

• He has a British passport. He must be British.
• He doesn’t speak a word of English. He can’t be British.

Can’t vs Mustn’t

Many learners confuse can’t and mustn’t.

For deduction, use can’t when you are sure something is not true.

• He can’t be at work. I’ve just seen him in the supermarket.

Use mustn’t for rules and prohibition.

• You mustn’t park here. It’s not allowed.

So, for deduction, we usually say:

Correct: She can’t be the manager. She looks too young.

Not: She mustn’t be the manager.

May, Might and Could for Deduction

We use may, might and could when something is possible, but we are not certain.

They are weaker than must.

• She isn’t answering her phone. She might be busy.
• He may know the answer, but I’m not sure.
• They could be waiting outside.

All three suggest possibility.

may	perhaps
might	perhaps
could	perhaps / it is possible

Is There a Difference Between May, Might and Could?

The difference is usually small.

May can sound slightly more formal.

• She may be at work.

Might is very common in everyday English.

• She might be at work.

Could often suggests one possible explanation among several.

• She could be at work, or she could be at the gym.

In many everyday situations, you can use may, might or could with a similar meaning.

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Modal Verbs of Deduction with Continuous Forms

When we want to talk about something happening now, we can use:

modal verb + be + -ing

• She must be working. She hasn’t replied to my message.
• They might be having dinner.
• The children can’t be sleeping. I can hear them laughing.

Compare:

• She must work from home.
  = I think this is generally true.
• She must be working from home today.
  = I think this is happening now.

Modal Verbs of Deduction in the Past

To make deductions about the past, use this structure:

modal verb + have + past participle

• She must have left early.
• He might have forgotten the meeting.
• They could have missed the train.
• She can’t have stolen the money.
• He may have gone home.

This structure is very important for B2 and C1 learners.

Must Have + Past Participle

Use must have + past participle when you are almost certain something was true or happened in the past.

• The floor is wet. Someone must have spilt water.

Might Have, May Have and Could Have + Past Participle

Use might have, may have or could have + past participle when something was possible in the past, but you are not certain.

• She might have missed the bus.
• He may have forgotten your email.
• They could have gone to the wrong address.

Can’t Have + Past Participle

Use can’t have + past participle when you are almost certain that something did not happen.

• He can’t have forgotten the meeting. I reminded him this morning.

In British English, can’t have is very common. You may also hear couldn’t have with a similar meaning.

• He couldn’t have known about the problem. Nobody told him.

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Present vs Past Deduction

This table shows the difference clearly.

Present	must + infinitive	She must be tired.
Past	must have + past participle	She must have been tired.
Present	might + infinitive	He might know her.
Past	might have + past participle	He might have known her.
Present	could + infinitive	They could be at home.
Past	could have + past participle	They could have been at home.
Present	can’t + infinitive	It can’t be true.
Past	can’t have + past participle	It can’t have been true.

Modal Verbs of Deduction Exercise

Download Modal Verbs of Deduction Practice Worksheet in PDF here 

Preparing for B2 First? Practise More Grammar!

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