Speculation & deduction: modal verbs— B2 Grammar Exercises
Published March 25, 2026
Exercise 1 — Gap Fill Select
He be at home now; his car is in the driveway.
She have forgotten about the meeting; she’s not here yet.
They be tired after such a long journey.
He have left early because I didn’t see him at the party.
She be joking; that story sounds unbelievable.
They have arrived by now; the train was delayed.
You be joking! That’s incredible news!
He have taken your book by mistake; he looks confused.
She have forgotten your birthday; she seemed very busy last week.
He be the new manager; I heard he was promoted last week.
Your colleague isn't at her desk and her coat is gone. You don't know where she is, but you can guess. She must have left early. She might be at lunch. She can't be in a meeting; her calendar's clear. Three guesses, three degrees of certainty, all built from modal verbs. This is how English handles speculation and deduction: not with separate verbs for "I'm sure" and "maybe," but with a small set of modals that signal exactly how confident the speaker is.
The modals divide cleanly into three certainty levels: certain, possible, and impossible — and each level has two forms: one for the present, one for the past.
The certainty scale
The modal you choose tells the listener how sure you are. Get this scale right and everything else falls into place.
| Certainty | Modal | Present | Past |
|---|---|---|---|
| strong certainty (positive) | must | must + base verb | must have + past participle |
| possible | may / might / could | may/might/could + base verb | may/might/could have + past participle |
| strong certainty (negative) | can't / couldn't | can't + base verb | can't/couldn't have + past participle |
Strong certainty: must and can't
Use must when the evidence makes you almost sure something is true. Use can't when the evidence makes you almost sure something is not true. The two are opposites; must not doesn't work as the negative here, as that means something different (prohibition).
Present situations
- The lights are on. They must be home.
- She speaks four languages, so she must have a real talent for it.
- You can't be hungry. You just ate.
- That can't be Daniel. He's in Madrid this week.
Past situations
- The grass is wet. It must have rained last night.
- Anna didn't reply, so she must have been busy.
- You can't have seen Tom yesterday. He was in hospital.
- They couldn't have known about the change; nobody told them.
Can't have and couldn't have are interchangeable for past deduction. Couldn't have sounds slightly softer.
Possibility: may, might, could
Use these when you think something is possible but you're not sure. All three are close in meaning. Might is the most common in everyday speech, may is slightly more formal, could emphasises that something is one option among several.
Present and future possibility
- He's not answering his phone. He might be in a meeting.
- The package may arrive tomorrow.
- That could be the postman; I heard the gate.
- She might not be coming; she didn't reply to the invite.
Past possibility
- She didn't show up. She may have forgotten.
- I can't find my keys. I might have left them at the café.
- The flight could have been delayed, so check the board.
- He may not have received your message; the server was down.
Register: may, might, could
The three modals overlap in meaning but not in tone. May sits at the formal end and dominates academic and professional writing. Might is the everyday default in speech and informal writing. Could often suggests one option among several being considered.
| Register | Example |
|---|---|
| formal / written | The applicant may have misunderstood the question. |
| everyday speech | He might've misread it. |
| neutral / weighing options | He could have misread it, or he could have skipped that section. |
Continuous forms
When the action is in progress, use the continuous form of the modal. This is especially useful for guessing what someone is doing right now or was doing at a specific past moment. All the speculation modals take the continuous, not just must.
| Time | Form | Example |
|---|---|---|
| Present continuous | modal + be + -ing | He must be working from home today. |
| Present continuous | modal + be + -ing | She may be working late; her light's still on. |
| Present continuous | modal + be + -ing | They might be sleeping; don't call yet. |
| Past continuous | modal + have been + -ing | You look tired. You must have been studying all night. |
| Past continuous | modal + have been + -ing | He could have been waiting for hours. |
Asking speculative questions
Speculation isn't only for statements. You can also wonder out loud, and the question forms shift the modal preferences. Could and can dominate questions; might is used but less commonly; must almost never appears in deductive questions.
- Where could she be? (Present — wondering about a location.)
- Can he be serious? (Present — expressing disbelief.)
- What might have happened? (Past — wondering about a cause.)
- Could they have forgotten the meeting? (Past — considering a possible explanation.)
Can in questions like Can he be serious? usually carries surprise or doubt — it's not a question expecting yes or no, more an expression of "really?"
Should for expected outcomes
There's a fourth flavour of deduction that doesn't fit the certainty scale neatly: deduction based on what's expected rather than what evidence directly shows.
- She left at six, so she should be home by now.
- The post should have arrived this morning.
Should here doesn't mean obligation — it means "this is what I expect to be true based on normal circumstances." Use it when reasoning from schedule, routine, or general knowledge rather than direct evidence.
Common mistakes
She can't be at home — the lights are off.
Mustn't means "is not allowed to," not "must not be true." For negative deduction, use can't.
He must have left already.
For past deduction, you need have + past participle after the modal — not the past tense alone.
It might have rained last night.
Same rule. After might, may, could, and must, the past is built with have + past participle.
She must have been tired.
Must've sounds like must of in fast speech, but in writing it's always have.
They could be at the office.
Can is rarely used for present speculation in positive sentences — use could, may, or might. Can only fits in questions (Where can he be?) and negatives (can't).
He might not have heard the alarm.
For negative past possibility, put not after the modal: might not have, may not have. Couldn't have means something stronger — it implies impossibility, not just doubt.
Same evidence, different modal: a worked example
The modal shifts the meaning even when the facts don't change. Five readings of the same scene.
| Evidence | Deduction | Speaker's certainty |
|---|---|---|
| Your friend's car is in the driveway. | She must be home. | I'm sure. |
| Your friend's car is in the driveway. | She may be home. | It's possible (slightly formal tone). |
| Your friend's car is in the driveway. | She might be home. | It's possible — maybe she got a lift somewhere instead. |
| Your friend's car is in the driveway, but you saw her at the airport. | She can't be home — I just saw her boarding. | I'm sure it's not true. |
| Her car is in the driveway and you know she was meant to fly out yesterday. | She couldn't have caught her flight, then. | I'm sure that earlier event didn't happen. |
Quick summary
- Three certainty levels: must (sure yes), might/may/could (possible), can't/couldn't (sure no).
- Present deduction: modal + base verb. Past deduction: modal + have + past participle.
- Never use mustn't for negative deduction — use can't.
- Write have, not of, even when it sounds like 've.
- For an action in progress, add be + -ing: must be working, might have been waiting.
- Use should when the deduction is based on expectation rather than direct evidence.
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