Modals of deduction in the present
Must, might and can't to say what you think is true now: she must be tired, he can't be at home.
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When we don't know a fact but have a reason to think something, we use a modal verb: must when we are sure it is true, can't when we are sure it is not true, and might, may or could when it is possible. This is called deduction.
How sure are you?
How sure | Modal | Example |
|---|---|---|
sure it's true | must | The lights are off. They must be out. |
it's possible | might, may, could | They might be at the cinema. |
sure it isn't true | can't (or couldn't) | They can't be asleep. It's only eight. |
The verb after the modal has no to and no ending: modal + be, or modal + another verb.
- You've been travelling all day. You must be tired.
- She might know the answer. Ask her.
- That can't be Pavel. He's in Berlin this week.
Must: I'm sure it's true
Use must when the facts make you almost certain.
- He's got three jobs. He must work very hard.
- Their house is enormous. They must have a lot of money.
- "I've just run ten kilometres." "You must be hungry."
- Incorrect: She must to be at work. Her car isn't here.
- Correct: She must be at work. Her car isn't here.After must, use the verb without to.
Can't: I'm sure it isn't true
The opposite of must for deduction is can't, not mustn't. Mustn't means "it's not allowed".
- That can't be right. Two and two don't make five.
- You can't be serious. It's minus ten outside.
- Incorrect: He mustn't be the manager. He looks about eighteen.
- Correct: He can't be the manager. He looks about eighteen.For "I'm sure it isn't true", use can't. Mustn't is for rules.
Couldn't is also possible here, with much the same meaning: That couldn't be true.
Might, may, could: it's possible
When you are not sure, use might, may or could. The negative is might not or may not.
- "Whose bag is this?" "It might be Olga's. She was sitting here."
- He may not know the address. Send it to him.
- Incorrect: I'm not sure, but she must be Polish.
- Correct: I'm not sure, but she might be Polish.If you're not sure, it's only possible, so use might, may or could.
Must be doing: an action now
For something in progress now, use the modal + be + -ing.
- Paul's coat is wet. It must be raining.
- Listen to that noise. The neighbours might be having a party.
- They can't be sleeping. All the lights are on.
- Incorrect: Lena isn't answering her phone. She must drive.
- Correct: Lena isn't answering her phone. She must be driving.For an action in progress now, use must be + -ing.
Two meanings of must
The same word can be a rule or a deduction. The situation tells you which.
- You must wear a helmet. (A rule: it's necessary.)
- You must be Anna's sister. You look just like her. (A deduction: I'm sure.)
Have to is mostly used for rules. For deduction in the present, use must.
Questions
In questions about what is possible, use could or might, or a question with do you think.
- Could this be the right building?
- Who might that be at the door?
- Do you think she's still at the office?
Deduction about the past (She must have forgotten) has its own page at B2, and the B2 quiz below practises both.