Modals of Deduction: Must, Might, Could and Can't
The lights are on, so they must be home. That can't be Sarah; she's in Paris. We use must, might, could and can't to guess about the present when we do not know for sure. Here is how sure each one is and how to use it correctly.
By LearnLingo Team · Updated
Form
| Modal | How sure | Example |
|---|---|---|
| must | about 95%: I'm almost sure it's true | You must be hungry after that long walk. |
| may | about 50%: it's possibly true | She may be in a meeting. |
| might | about 50%, often a little less sure | He might be at the gym. |
| could | about 50%: it's possible | The keys could be in the car. |
| might not, may not | about 50%: possibly not true | They might not know about the change. |
| can't (couldn't) | about 95%: I'm almost sure it's not true | That can't be Tom. He's on holiday. |
The percentages are only a guide to help you remember the strength. In real speech, may, might and could are very close in meaning.
| Pattern | Use | Example |
|---|---|---|
| modal + base verb | states and general situations now | He must know the answer. |
| modal + be + adjective or noun | describing a person or situation | She can't be serious. |
| modal + be + -ing | an action in progress now | They might be sleeping. |
| modal + have + past participle | a guess about the past (B2) | She must have forgotten. |
Modals never take -s and are never followed by to: he must be, not he musts be or he must to be.
| Meaning | Use | Don't use for this meaning |
|---|---|---|
| almost sure it's true | It must be expensive. | It can be expensive. (This means it is sometimes expensive.) |
| almost sure it's not true | It can't be expensive. | It mustn't be expensive. |
| possibly not true | It might not be expensive. | It can't be expensive. (This is much stronger.) |
In deduction, the negative of must is can't, not mustn't. Mustn't means it is not allowed. In American English, must not is sometimes used for deduction: He must not be home.
Must: when you are almost sure
Use must when you have strong evidence and you are almost certain something is true. You are not 100% sure, because you did not check, but the logic is clear.
This must is different from the must of obligation (You must wear a seatbelt). Here it is about what you believe, not about rules. Compare: You must be tired (I'm sure you are tired) and You must go to bed (it's necessary). For obligation, see have to and must.
A very common use is reacting to what someone tells you: I've been travelling for 20 hours. You must be exhausted!
The lights are on. They must be at home.
You must be Anna. Nice to meet you!
A friendly way to greet someone you are expecting.
He runs every morning. He must be very fit.
You've worked all day. You must be hungry.
She speaks perfect French. She must have lived in France.
Must have + past participle is a guess about the past.
Might, may and could: when it is possible
Use might, may or could when something is possibly true but you are not sure. They have very similar meanings in this use. May is a little more formal, and might is the most common in everyday speech.
For the negative, use might not or may not: He might not know. Be careful with couldn't: it is not like might not. Couldn't means almost the same as can't (impossible): It couldn't be him.
Could is also used for ability and requests, so context matters. For those meanings, read can and could.
Where's Leyla? She might be in the library.
Take an umbrella. It may rain later.
Don't eat that mushroom. It could be poisonous.
I'm not sure, but he might not have my number.
The parcel may arrive tomorrow or on Friday.
Can't: when you are almost sure it is not true
Use can't when you have strong evidence that something is not true or is impossible. It is the opposite of must in deduction.
Couldn't is also possible and sounds a little less direct: That couldn't be right. Do not use mustn't here. Mustn't means something is not allowed: You mustn't smoke here.
That can't be Ali. He's in Germany this week.
You can't be serious! You want to quit your job?
This bill can't be right. We only had two coffees.
She can't be hungry. She's just eaten a huge lunch.
Actions happening now: modal + be + -ing
When you guess about an action that is in progress now, use the modal + be + the -ing form. This is the continuous form of the modal.
Compare: He must work hard (in general, it's part of his life) and He must be working hard (right now or these days). The phone isn't answered: She might be driving. Loud music from next door: They must be having a party.
There's music next door. They must be having a party.
Kerem isn't answering. He might be driving.
You can't be studying. Your book is closed!
Shh, the baby could be sleeping.
Talking about the past
To guess about the past, use the modal + have + past participle: must have, might have, could have, can't have, couldn't have. For example: The ground is wet. It must have rained in the night. She didn't reply. She might not have seen my message.
This pattern is part of the B2 topic modal perfects, where you can learn it in detail, including should have and would have. At B1, it is enough to recognise it and use the most common forms.
I can't find my wallet. I must have left it at the café.
He can't have finished already. He only started ten minutes ago.
They might have missed the bus.
In real life
People use these modals all the time to react, to guess and to sound less direct. You must be joking! and You can't be serious! are common reactions to surprising news. That must be nice is a polite reply when someone tells you about a holiday or a new job.
In speech, people also use other words to show how sure they are: probably, maybe, perhaps, I bet, I doubt it. Maybe he's at home means the same as He might be at home, but note that maybe goes at the start of the sentence, not after might. For other polite ways to soften what you say, see polite requests in English.
You won the lottery? You must be joking!
A month in Bali? That must be amazing.
Is that the delivery? It might be. I'll check.
In short answers we can end with the modal + be.
Who's at the door? It can't be the pizza already.
Common mistakes
He mustn't be at home. His car isn't here.
He can't be at home. His car isn't here.
In deduction, the opposite of must is can't. Mustn't means not allowed.
She must to be tired.
She must be tired.
Modal verbs are followed by the base verb with no to.
They might are at the cinema.
They might be at the cinema.
After a modal, use the base form be, not am, is or are.
He must work now; his office light is on.
He must be working now; his office light is on.
For an action in progress at this moment, use modal + be + -ing.
Maybe she might be ill.
She might be ill. or Maybe she's ill.
Maybe and might both show possibility, so using both is repetitive. Choose one.
Practice quiz
10 questions. Pick an answer to see if it is right and why.
1. You've been working for twelve hours. You ___ be exhausted.
2. That ___ be Mehmet. He's in Japan this month.
3. Choose the correct sentence.
4. I'm not sure where Can is. He ___ be in the garden.
5. Nobody is answering the phone. They ___.
6. What does It may not be true mean?
7. The streets are wet. It ___ rained in the night.
8. Which sentence is about obligation, not deduction?
9. This answer ___ be right. Two plus two isn't five!
10. Take your coat. It ___ get cold later.
0 of 10 answered
Show all answers
- must
- can't
- She might be at work.
- could
- must be sleeping
- It is possibly false.
- must have
- You must wear a helmet.
- can't
- might
See what the B1 English course covers, or test your level first.
People also ask
What are modals of deduction?+
They are modal verbs we use to make logical guesses: must (almost sure it's true), might, may and could (possibly true), and can't or couldn't (almost sure it's not true). They show how certain the speaker is, based on evidence.
What is the difference between must and might?+
Must shows strong certainty based on evidence: He must be rich, look at his car. Might shows that something is only possible: He might be rich, I don't know. Must is about 95% sure; might is about 50%.
Why can't I say mustn't for deduction?+
In British English, mustn't means something is not allowed, so it is not used for logical guesses. Use can't instead: It can't be true. In American English, must not is sometimes used for deduction (He must not be home), but can't is correct everywhere.
How do I make deductions about the past?+
Use the modal + have + past participle: She must have left, they might have forgotten, he can't have seen us. This is covered in detail in the modal perfects topic.
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