Modal Verbs of Deduction
We use modal verbs to make deductions — logical conclusions based on evidence. 'Must' expresses near certainty that something is true. 'Can't' (or 'couldn't') expresses near certainty that something is NOT true. 'Might' or 'could' expresses a possibility — we are not sure.
For present deductions: must / can't / might + base verb ("She must be tired."). For past deductions: must / can't / might + have + past participle ("She must have been tired yesterday.").
The strength of certainty: must (almost certain positive) → might / could (possible) → can't (almost certain negative). None of these express 100% certainty — they are all conclusions from evidence.
| Certainty | Modal | Example |
|---|---|---|
| Almost certain (positive) | must + base verb | He must be at home — his car is here. |
| Possible | might / could + base verb | She might be in a meeting. |
| Almost certain (negative) | can't + base verb | They can't be asleep — the lights are on. |
| Past — almost certain | must have + past participle | He must have left early. |
| Past — possible | might have + past participle | She might have missed the train. |
| Past — almost certain negative | can't have + past participle | They can't have eaten already. |
⚠ Don't confuse 'must' (deduction: 'He must be cold') with 'must' (obligation: 'You must wear a seatbelt').
⚠ Past deductions always use: modal + HAVE + past participle — never modal + past simple.
- She isn't answering her phone — she must be busy.
- He's been awake since 4am. He must be exhausted.
- That can't be right — the answer was 42, not 24.
- They left three hours ago — they might have arrived by now.
- She studied for months. She can't have failed.
- Nobody's home. They must have gone away for the weekend.
- I'm not sure where he is — he could be at the gym.
- The window is open and the keys are missing. Someone must have been here.
- She must have tired after the marathon. → She must have been tired after the marathon. Past deduction: must have + past participle. 'Tired' is an adjective, so the past participle of 'be' ('been') is required.
- He can't have not been at the office. → He can't have been at the office. Double negative: 'can't' is already negative — don't add 'not have'.
- They might arrived late. → They might have arrived late. Past deduction = might HAVE + past participle. Don't omit 'have'.
- She must be left her bag on the train. → She must have left her bag on the train. Past deduction needs 'have' after the modal. 'Must be left' mixes present and past forms.
Choose the correct modal verb to complete each sentence.
- The lights are off and the door is locked. They _____ (must / can't) be home.
- She's been training every day for a year — she _____ (must / might) be very fit.
- He _____ (can't / might) have eaten the cake — he's allergic to chocolate.
- I haven't seen her today. She _____ (could / must) be working from home.
- They _____ (must / can't) have left yet — their coats are still here.
Each sentence contains a mistake. Find the mistake and rewrite the sentence correctly.
- She must arrived early — the door is already open.
- He can't has been at the party — he was abroad.
- They must be not sleeping — I can hear the TV.
- It might be raining earlier — the ground looks wet.
Look at each situation. Write a deduction using must, might, or can't.
She has dark circles under her eyes. → She must have slept badly last night.
- His phone has been off for two days.
- There's a half-eaten pizza and an open laptop on the table.
- She scored 98% on her exam.
- The café is completely empty, even though it's lunchtime.
- He arrived at work soaking wet.
Use the information to write a deduction. Use must have, can't have, or might have.
She left her umbrella at home. It started raining. → She must have got wet.
- He studied for six months. He took the exam this morning.
- She said the food was terrible. She left most of it on the plate.
- They booked the hotel for two weeks, but they left after three days.
- The safe was open and the money was gone.
Complete using must, can't, or might + the correct form of the verb in brackets.
- The floor is wet — someone _____ (spill) something.
- He _____ (know) about it — nobody told him.
- She's very knowledgeable about history — she _____ (be) a teacher.
- They ordered pizza thirty minutes ago — it _____ (arrive) soon.
- The ticket _____ (be) free — the queue is enormous.
- He _____ (forget) his password again — he looks confused.
- I can smell something burning — someone _____ (leave) the oven on.
- She _____ (take) the wrong train — she's still not here.
Discuss these mystery situations with a partner. Use must, might, and can't to make deductions.
- You come home to find your front door open, a half-eaten sandwich on the table, and a book left face-down on the sofa. What do you think happened?
- Your friend hasn't replied to your messages in three days. What could have happened?
- A very expensive painting has disappeared from a locked gallery. What are some possible explanations?
- You find an old, unopened letter addressed to you in a drawer. What might it contain?
For each situation, try to make at least three deductions — one with 'must', one with 'might', and one with 'can't'. Add supporting evidence for each one.