Modal Verbs of Deduction in English: Must, Might, May, Could and Can’t

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.

Have you ever looked at a situation and made a guess based on what you could see?

For example:

• The lights are off. They must be asleep.
• She isn’t answering her phone. She might be busy.
• That can’t be David. He’s on holiday in Spain.

These are examples of modal verbs of deduction.

We use modal verbs of deduction when we want to say how sure we are about something. We are not simply guessing wildly; we are using the information we have to make a logical conclusion.

In this lesson, you will learn how to use must, might, may, could and can’t for deduction in the present and the past, with clear examples, common mistakes and practice exercises.

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.

Do not use to after the modal verb.

Incorrect:

• She must to be tired.

Correct:

• She must be tired.

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.

Examples:

• She has been working all day. She must be exhausted.
• The kitchen smells wonderful. Someone must be cooking dinner.
• He drives a very expensive car. He must earn a lot of money.
• You’ve been studying English for years. You must know a lot of grammar by now.
• The streets are wet. It must be raining or it must have rained recently.

In these sentences, the speaker is not 100% certain, but they are very confident.

More examples with must

• Sarah is not in her office. She must be in a meeting.
• He knows a lot about computers. He must work in IT.
• They look very relaxed. They must be enjoying their holiday.
• The baby is crying. She must be hungry.
• This restaurant is full every evening. The food must be excellent.

Can’t for Deduction

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

Examples:

• That can’t be John. John is much taller.
• She can’t be at home. Her car isn’t outside.
• He can’t know the answer. He wasn’t in the lesson.
• They can’t be married. They’ve only just met.
• This story can’t be true. It sounds completely impossible.

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.

Of course, real life is sometimes more complicated than grammar examples — but the grammar rule still stands politely in the corner, doing its job.

Important: 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.

Example:

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

Use mustn’t for rules and prohibition.

Example:

• 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.

Examples:

• 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.
• The noise might be coming from the neighbours’ flat.
• This email could be important, so don’t delete it.

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.

Modal Verbs of Deduction with Continuous Forms

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

modal verb + be + -ing

Examples:

• She must be working. She hasn’t replied to my message.
• They might be having dinner.
• He could be waiting outside.
• The children can’t be sleeping. I can hear them laughing.
• Someone may be using the computer at the moment.

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

Examples:

• 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.

Examples:

• The floor is wet. Someone must have spilt water.
• I called her three times, but she didn’t answer. She must have been busy.
• He passed the exam with a very high mark. He must have studied hard.
• The children are covered in mud. They must have been playing outside.
• I can’t find my keys. I must have left them at work.

In each sentence, the speaker is using present evidence to make a logical conclusion about the past.

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.

Examples:

• She might have missed the bus.
• He may have forgotten your email.
• They could have gone to the wrong address.
• I might have left my phone in the car.
• The package may have arrived while we were out.

These forms are useful when you are not sure what happened.

More examples

• Tom isn’t here yet. He might have got stuck in traffic.
• I can’t find the document. I may have deleted it by mistake.
• Nobody answered the door. They could have gone out.
• She looked upset yesterday. Something might have happened at work.

Can’t Have + Past Participle

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

Examples:

• He can’t have forgotten the meeting. I reminded him this morning.
• She can’t have taken your bag. She left before you arrived.
• They can’t have finished the project already. They only started yesterday.
• You can’t have seen Mark in London. He’s been in Paris all week.
• The shop can’t have closed yet. It’s only five o’clock.

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

Example:

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

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: Examples in Context

Let’s look at a short situation.

You arrive at your friend Anna’s house. Her car is outside, but nobody answers the door. You can hear music inside.

You might say:

• She must be at home. Her car is outside.
• She might be having a shower.
• She could be listening to music and not hear the doorbell.
• She can’t be sleeping because the music is very loud.
• She must have forgotten that we were meeting today.

This is how modal verbs of deduction work in real life. You look at the evidence and decide what is likely, possible or impossible.

Common Mistakes with Modal Verbs of Deduction

Mistake 1: Using “to” After a Modal Verb

Incorrect:

• She must to be tired.
• He might to know the answer.

Correct:

• She must be tired.
• He might know the answer.

After modal verbs, use the infinitive without to.

Mistake 2: Forgetting “Have” in Past Deduction

Incorrect:

• She must left early.
• He might forgotten the meeting.

Correct:

• She must have left early.
• He might have forgotten the meeting.

For past deduction, remember:

modal verb + have + past participle

Mistake 3: Using “Mustn’t” Instead of “Can’t”

Incorrect:

• He mustn’t be at home. His car isn’t outside.

Correct:

• He can’t be at home. His car isn’t outside.

Use can’t when you are sure something is not true.

Use mustn’t when something is not allowed.

Mistake 4: Confusing Maybe and May Be

Maybe is one word. It is an adverb meaning perhaps.

Example:

• Maybe she is at work.

May be is two words: modal verb + verb.

Example:

• She may be at work.

Both sentences have a similar meaning, but the grammar is different.

More examples:

• Maybe he is ill.
• He may be ill.
• Maybe they forgot.
• They may have forgotten.

Cambridge Exam Tip: Modal Verbs of Deduction

Modal verbs of deduction are especially useful for Cambridge B2 First and C1 Advanced.

You may need them in:

• sentence transformations
• Use of English tasks
• speaking tasks
• essay writing
• story writing
• reports and reviews

They are particularly common in sentence transformations.

Examples:

I’m sure she is tired.

She must be tired.

Perhaps he missed the train.

He might have missed the train.

I’m sure this isn’t the right address.

This can’t be the right address.

I’m sure they didn’t see us.

They can’t have seen us.

Perhaps she left her phone at home.

She may have left her phone at home.

Using these structures accurately can make your English sound more natural, precise and advanced.

Quick Practice: Modal Verbs of Deduction

Complete the sentences:

1. She isn’t answering her phone. She ______ be busy.
2. He drives a very expensive car. He ______ earn a lot of money.
3. That ______ be Tom. Tom is much taller.
4. I can’t find my keys. I might ______ left them at work.
5. They left five minutes ago, so they can’t ______ far.

Answer key: 1. might, 2. must, 3. can’t, 4. have, 5. be

Want more practice? Download the Modal Verbs of Deduction Practice Worksheet and practise must, might, may, could and can’t in present and past deduction.

Final Thoughts

Modal verbs of deduction help you express how sure you are about something. Instead of saying only “maybe” or “I’m sure”, you can use more precise grammar.

Use must when you are almost certain something is true.

• She must be tired.

Use may, might or could when something is possible.

• She might be tired.

Use can’t when you are almost certain something is not true.

• She can’t be tired. She has just slept for ten hours.

For past deduction, remember the structure:

modal verb + have + past participle

• She must have forgotten.
• He might have missed the train.
• They can’t have known about the problem.

Once you learn these structures, your English becomes much more flexible. You can speculate, explain, guess, doubt and conclude — all with just a few modal verbs doing a very respectable amount of work.

FAQ 

What are modal verbs of deduction?

Modal verbs of deduction are modal verbs we use to make logical guesses based on evidence. For example, She must be tired means I am almost sure she is tired.

What is the difference between must and might?

Use must when you are almost sure something is true. Use might when something is possible but not certain.

What is the past form of modal verbs of deduction?

For past deduction, use modal verb + have + past participle. For example: She must have forgotten, He might have left, They can’t have known.

Can I use mustn’t for deduction?

Usually, no. For deduction, use can’t when you are sure something is not true. Mustn’t normally means something is not allowed.

Related posts:

Modal Verbs in English

Modal Verbs May and Might

Can, Could, Be Able to

Sentence Transformations for B2 First 

Present Perfect Simple in English 

Narrative Tenses in English 

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