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

If you want to sound more natural and precise in English, you need to learn modal verbs of speculation. We use them to guess, deduce, and talk about possibilities in the present and the past. In this lesson, you will learn how to use must, may, might, could, can’t, and couldn’t with clear explanations, natural example sentences, common mistakes, and useful practice.

Let’s get started.

What are modal verbs of speculation?

Modal verbs of speculation are modal verbs we use when we want to say that something is possible, probable, or impossible.

They show how certain we feel about something.

For example:

• She must be at work.
  = I am quite sure she is at work.
• He might be tired.
  = It is possible that he is tired.
• They can’t be serious.
  = I am sure they are not serious.

We often use these forms when we:

• make guesses
• draw conclusions from evidence
• imagine possible explanations
• comment on situations we do not know for certain

The most common modal verbs of speculation

The main modal verbs used for speculation are:

• must
• may
• might
• could
• can’t
• couldn’t

Let’s look at them one by one.

1. Must for strong belief or logical conclusion

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

Structure

must + base verb

must + be + noun/adjective/prepositional phrase

Examples

• She has been studying all day. She must be exhausted.
• The lights are off. They must be asleep.
• He speaks flawless Japanese. He must have lived in Japan for a long time.
• You’ve been walking for hours. You must need a rest.

Here, must does not express obligation. It expresses a strong conclusion based on what we know.

Compare

• You must finish your homework. = obligation
• You must be tired. = speculation

That difference is very important.

2. May, might, and could for possibility

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

Structure

may / might / could + base verb

may / might / could + be + noun/adjective

Examples

• It may rain later, so take an umbrella.
• She might know the answer.
• They could be on the train.
• He may be ill.
• This story might be true.
• We could be wrong.

These three modals are often very similar in meaning.

Is there a difference?

In many everyday situations, may, might, and could can all be used to express possibility.

• He may be at home.
• He might be at home.
• He could be at home.

All three mean: it is possible.

However, many learners feel that:

• may sounds a little more formal
• might sounds a little more uncertain
• could often suggests one possible explanation among others

This is not a hard rule, but it is a useful feeling to keep in mind.

3. Can’t and couldn’t for negative deduction

We use can’t or couldn’t when we believe something is impossible.

Structure

can’t / couldn’t + base verb

can’t / couldn’t + be + noun/adjective

Examples

• She can’t be serious.
• He couldn’t know that already.
• They can’t be at home. Their car isn’t outside.
• This couldn’t be the right address.
• You can’t be hungry. You’ve just had lunch.

These forms express strong disbelief.

Important

For speculation, English normally uses can’t much more often than mustn’t.

• He can’t be at work. ✅
• He mustn’t be at work. ❌

Why? Because mustn’t usually means prohibition, not negative speculation.

• You mustn’t smoke here. = it is forbidden

So when you want to say something is impossible, use can’t or couldn’t, not mustn’t.

Modal verbs of speculation about the present

When we speculate about the present, we usually use:

modal + base verb

or

modal + be + complement

Examples

• She must feel nervous.
• They might arrive soon.
• He could be in his office.
• You can’t mean that.
• This may seem strange at first.
• They must be very wealthy.

We use these forms when we are talking about what is true now.

Modal verbs of speculation about the past

When we speculate about the past, we usually use:

modal + have + past participle

This structure is extremely important.

Examples

• She must have forgotten our meeting.
• They may have missed the bus.
• He might have gone home early.
• We could have taken the wrong road.
• She can’t have seen the message.
• He couldn’t have done this alone.

These forms help us guess or deduce what happened earlier.

Present vs past speculation

Compare these pairs carefully:

Present

• She must be tired.
  = I think she is tired now.

Past

• She must have been tired.
  = I think she was tired earlier.

Present

• They might be at home.
  = Perhaps they are at home now.

Past

• They might have been at home.
  = Perhaps they were at home earlier.

The difference is small in form but very important in meaning.

Quick meaning guide

You can think of the modals like this:

• must = I am almost sure
• may / might / could = it is possible
• can’t / couldn’t = I am sure it is not true

That is the heart of the system.

Examples in everyday situations

Let’s look at modal verbs of speculation in natural context.

At home

• Someone’s at the door. It must be the postman.
• I can’t find my keys. I might have left them in the kitchen.
• The house is dark. They may be out.
• That noise could be the washing machine.

At work

• She isn’t answering her phone. She must be in a meeting.
• He looks worried. He might have heard some bad news.
• They can’t be ready yet. We only gave them the brief this morning.
• The manager could have forgotten to send the email.

While travelling

• We’ve been waiting for ages. The train must be delayed.
• They might have taken the wrong platform.
• This can’t be the right station.
• We could be closer than we think.

Modal verbs of speculation in Cambridge exams

These verbs are especially useful in Cambridge English exams because they help you:

• sound more natural in speaking
• speculate in photo tasks
• express uncertainty in discussions
• write in a more flexible and advanced way

Useful speaking phrases

• He might be feeling stressed.
• They could be discussing a problem.
• She may have forgotten something important.
• It must be difficult for them.
• This can’t be an easy situation.

These phrases are perfect for B2 First and C1 Advanced speaking tasks, especially when describing photos or discussing possibilities.

Common mistakes with modal verbs of speculation

Let us look at a few mistakes learners often make.

1. Using mustn’t instead of can’t

• He mustn’t be at home. ❌
• He can’t be at home. ✅

Use can’t for negative deduction.

2. Using the wrong form after a modal

• She might to be tired. ❌
• She might be tired. ✅

After a modal verb, use the base form of the verb.

3. Forgetting have in past speculation

• He might went home. ❌
• He might have gone home. ✅

For past speculation, use:

modal + have + past participle

4. Confusing obligation with speculation

• You must finish this report today. = obligation
• You must be very busy today. = speculation

The same modal can have different meanings depending on context.

5. Overusing only maybe

Many learners say maybe again and again:

• Maybe he is tired.
• Maybe she forgot.
• Maybe they missed the train.

This is not wrong, but modal verbs sound more natural and more advanced:

• He might be tired.
• She may have forgotten.
• They could have missed the train.

A little variety goes a long way.

A simple rule to remember

When speculating:

About the present

Use

modal + base verb

or

modal + be

About the past

Use

modal + have + past participle

That is the golden rule.

Mini comparison chart

Strong certainty

• She must be at work.
• She must have left already.

Possibility

• She might be at work.
• She might have left already.

Impossibility

• She can’t be at work.
• She can’t have left already.

Beautiful little system, isn’t it? English does enjoy pretending to be chaotic before quietly becoming logical.

Final thoughts

Modal verbs of speculation help you do something very important in English: speak with nuance.

Instead of making flat, simple statements, you can express doubt, possibility, certainty, and deduction in a much more natural way.

That is exactly how real communication works. After all, in life we are not always completely sure. Sometimes we guess, sometimes we infer, and sometimes we are convinced.

And English has a lovely set of tools for that.

So the next time you want to say what you think probably happened, what might be true, or what simply cannot be possible, reach for a modal verb of speculation.

Your English will immediately sound more fluent, more precise, and much more natural.

Useful summary

• Use must for strong certainty.
• Use may, might, could for possibility.
• Use can’t, couldn’t for impossibility.
• Use modal + base verb for present speculation.
• Use modal + have + past participle for past speculation.

Modal Verbs of Speculation Extensive exercise worksheet for B2-C1 learnerss – download the PDF here

FAQ – modal verbs of speculation

What are modal verbs of speculation?
Modal verbs of speculation are verbs such as must, may, might, could, and can’t that we use to guess or deduce something.

What is the difference between must and might in speculation?
Must shows strong certainty, while might shows possibility.

How do we use modal verbs of speculation in the past?
We usually use modal + have + past participle, for example: She might have forgotten.

Is can used for speculation?
In affirmative speculation, English usually prefers could, may, or might. For negative deduction, we often use can’t.

Related posts:

Modal Verbs of Obligation

Modal Verb MUST

Expressing probability in English

Can, Could, Be Able to

Modal Verbs for Expectation

Talking about Unreal Situations in English

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