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Unit 11
Modal Verbs: Probability
might, could, must, cannot
Modals for Probability — Present & Future
Modal verbs let us express how certain or likely something is — right now or in the future. Instead of always using will, we use might, could, should, must and cannot to show different degrees of certainty.
Key rule: Modal verbs are always followed by the base form — no to, no -s, no -ing.
The Probability Scale
| Certainty level | Modal(s) | Meaning | Example |
|---|---|---|---|
| ~95% (positive) | must | Strong logical deduction — almost certain it IS true | He's been working all day — he must be exhausted. |
| ~95% (negative) | cannot / can't | Strong deduction — almost certain it is NOT true | She cannot be serious — that's impossible! |
| ~50–70% | should | Expected to be true / likely | The parcel should arrive tomorrow. |
| ~30–50% | might / could | Possible but not certain | It might rain later. / She could be at the gym. |
Must vs Cannot — Logical Deduction
Use must when evidence strongly suggests something IS true. Use cannot when evidence strongly suggests something is NOT true.
| Must (positive deduction) | Cannot (negative deduction) |
|---|---|
| His car is outside — he must be home. | His car is gone — he cannot be home. |
| She answered every question — she must know the topic. | She failed the test — she cannot know it well. |
| It's December — it must be cold. | It's 35°C — it cannot be cold. |
Might / Could — Possibility
- Might and could both express possibility — we are not sure.
- We might go to Italy next summer. (= perhaps, not decided)
- It could rain later — bring an umbrella. (= it's possible)
- She might be at the library. (= I'm not certain)
- Could can also suggest an alternative option: We could take the train instead.
Should — Expectation
Should expresses what is expected or likely to happen — not just advice.
| Advice (familiar) | Expectation / likelihood |
|---|---|
| You should see a doctor. | The results should be ready soon. |
| You should call her. | The train should arrive on time. |
| He should study more. | The meeting should finish by 3. |
Common Contrasts
| Sentence | Modal used | Degree of certainty |
|---|---|---|
| It will rain tomorrow. | will | ~100% — certain fact/strong prediction |
| It must be raining — the streets are wet. | must | ~95% — deduction from evidence |
| It should rain — the forecast said so. | should | ~70% — expected |
| It might / could rain later. | might/could | ~40% — possible |
| It cannot be raining — the sky is clear! | cannot | ~5% — almost impossible |
Negatives, Continuous Forms & Questions
| Point | ✅ Example | Note |
|---|---|---|
| Negative deduction = can't, not mustn't | He can't be hungry — he just ate. | |
| might not / may not = maybe not | She might not come. (= perhaps she won't) | Different from can't (= impossible). |
| Modal + be + -ing for now | He must be working — his door is closed. | an activity in progress now |
| Questions: Could / Might ...? | Could it be a mistake? | Don't use |
| may ≈ might | The shop may be closed on Sundays. | slightly more formal |
Annotated Examples
Subject
Verb
Auxiliary
Object
Adjective
Adverb
Preposition
Hover over any word to see its role.
subjectHeauxiliarymust beadjectivetired—he hasn't slept in two days.
↳ Must = strong deduction from evidence.
subjectItverbmight rainadverblater—bring an umbrella.
↳ Might = possibility, not certain.
subjectSheauxiliarycannot beprepositionat home—I just saw her downtown.
↳ Cannot = strong negative deduction.
Common Mistakes
It will can rain tomorrow.
→
It might rain tomorrow.
Never combine "will" with another modal — use "might" alone.
She must to be home.
→
She must be home.
No "to" after modal verbs — ever.
He musts be tired.
→
He must be tired.
Modal verbs never take -s for any subject.
It might to snow tonight.
→
It might snow tonight.
No "to" after modal verbs.
They coulds arrive soon.
→
They could arrive soon.
Modal verbs never take -s.
She mustn't be at home — her car isn't here.
→
She can't be at home — her car isn't here.
Negative deduction → can't.
He must work now — the light in his office is on.
→
He must be working now.
Activity in progress now → must be + -ing.