Modals for Probability: Must, Should, Might, May, Could, Can't
Grammar Rule in 30 Seconds
Use modal verbs to express your level of certainty about a situation based on evidence or logic.
- Use 'must' for near-certainty: 'He must be tired.'
- Use 'might/may/could' for possibility: 'She might be late.'
- Use 'can't' for near-certainty of impossibility: 'It can't be true.'
Modals of probability are not interchangeable — each signals a different level of confidence. Using the right one shows C1-level precision in reasoning and argument.
The Probability Scale
must be / can't be — logical conclusion
✅ No answer — she must be asleep.
✅ That can't be right — I just checked.
✅ He must have been travelling all night. (past)
should / ought to — expectation
✅ The results should be ready by now.
✅ She ought to have received it by now.
might / may / could — open possibility
✅ It could be a wiring issue.
✅ She may well be right. (may well = stronger)
✅ There is bound to be a simpler solution. (near-certain)
Modal Probability Formation
| Certainty | Present | Past |
|---|---|---|
|
High (Positive)
|
Must + Verb
|
Must + have + P.P.
|
|
High (Negative)
|
Can't + Verb
|
Can't + have + P.P.
|
|
Possibility
|
Might/May/Could + Verb
|
Might/May/Could + have + P.P.
|
Common Contractions
| Full Form | Contraction |
|---|---|
|
Cannot
|
Can't
|
|
Could not
|
Couldn't
|
|
Might have
|
Might've
|
Meanings
Modal verbs of probability are used to express how certain we are about a deduction or a situation.
Logical Deduction
Concluding something is true based on evidence.
“She's wearing a coat, so it must be cold.”
“He's not answering; he might be in a meeting.”
Possibility
Expressing that something is possible but not certain.
“It could rain later.”
“They may arrive by noon.”
Impossibility
Expressing that something is logically impossible.
“That can't be right.”
“It couldn't have been him.”
Reference Table
| Form | Structure | Example |
|---|---|---|
|
Affirmative (Present)
|
Subject + must + verb
|
He must be tired.
|
|
Affirmative (Past)
|
Subject + must + have + P.P.
|
He must have been tired.
|
|
Negative (Present)
|
Subject + can't + verb
|
He can't be tired.
|
|
Negative (Past)
|
Subject + can't + have + P.P.
|
He can't have been tired.
|
|
Possibility (Present)
|
Subject + might + verb
|
He might be tired.
|
|
Possibility (Past)
|
Subject + might + have + P.P.
|
He might have been tired.
|
Formality Spectrum
He may be in his office. (Workplace)
He might be in his office. (Workplace)
He's probably in his office. (Workplace)
He's likely at his desk. (Workplace)
Certainty Scale
High
- Must 100% sure
Possibility
- Might 50% sure
Impossibility
- Can't 0% sure
Examples by Level
It might rain.
He may be home.
That can't be right.
It must be cold.
They must have left.
She could be at work.
It can't be true.
He might have forgotten.
He must be working late again.
They might have been delayed by traffic.
That couldn't have been him; he's in London.
She may have already finished the report.
The project must have been completed by now.
It could be that they are waiting for us.
He can't have seen the email yet.
They might be considering our proposal.
He must have been working on this for weeks.
It could have been a misunderstanding, but I doubt it.
They may well have decided to postpone the event.
That can't have been the only reason for his departure.
One might have expected a more favorable outcome.
It could not but have been a deliberate choice.
They must be being watched by the authorities.
He may have been misinformed, or perhaps he lied.
Easily Confused
Learners mix up 'I must go' (obligation) and 'He must be home' (deduction).
Learners use 'mustn't' for negative deduction.
Learners think they have different meanings.
Common Mistakes
He must to be home.
He must be home.
He musts be home.
He must be home.
It mustn't be true.
It can't be true.
He might be go.
He might go.
He must have be home.
He must have been home.
He can be not home.
He can't be home.
It might be rain.
It might rain.
He must have went.
He must have gone.
He could have go.
He could have gone.
It must be have been.
It must have been.
He must have been being there.
He must have been there.
It might have been to be true.
It might have been true.
He can't have to be there.
He can't be there.
It must be that he has been.
He must have been.
Sentence Patterns
He ___ be at home.
They ___ have forgotten the meeting.
It ___ be true; I saw it myself.
She ___ have been working all night.
Real World Usage
She must be having a great time!
Might be late, sorry!
We could look into that strategy.
The flight might be delayed.
The driver must be lost.
These findings may suggest a correlation.
Context is King
Mustn't Trap
Use 'Can't' for Negative
Politeness
Smart Tips
Use 'might' to hedge your statement and sound more natural.
Use 'must' to show you have reached a logical conclusion.
Always use 'can't' instead of 'mustn't'.
Remember to add 'have' + past participle.
Pronunciation
Contractions
In spoken English, 'might have' often becomes 'might've' (/maɪtəv/).
Stress
Modals are usually unstressed unless you want to emphasize the certainty.
Falling
He MUST be home. ↘
Strong conviction.
Memorize It
Mnemonic
Must is a Must-have for certainty; Might is for Maybe.
Visual Association
Imagine a detective with a magnifying glass. If they see a footprint, they say 'It MUST be him!' If they see a shadow, they say 'It MIGHT be him.'
Rhyme
If you're sure, use must, if you're not, use might, if it's impossible, use can't to be right.
Story
Detective Miller looked at the empty safe. 'The thief must have had a key,' he said. 'He might have entered through the window,' his partner suggested. 'No, the window is locked; it can't have been the window.'
Word Web
Challenge
Look around your room and make 5 guesses about your belongings using different modals.
Cultural Notes
British speakers often use 'might' more frequently than 'may' to sound less formal.
Americans often use 'probably' instead of modals for probability in casual speech.
In academic writing, 'may' is preferred for hedging claims.
Modal verbs evolved from Old English preterite-present verbs.
Conversation Starters
Why is the office empty today?
Who do you think left this bag here?
Why didn't he show up to the meeting?
What do you think caused the market crash?
Journal Prompts
Common Mistakes
Test Yourself
He ___ be home; his car is in the driveway.
Which sentence is correct?
Find and fix the mistake:
It mustn't be true.
He must be tired. -> He ___ tired.
A: Why is the store closed? B: It ___ a holiday.
Match each item on the left with its pair on the right:
they / must / have / forgotten
I'm not sure, but it ___ rain.
Score: /8
Practice Exercises
8 exercisesHe ___ be home; his car is in the driveway.
Which sentence is correct?
Find and fix the mistake:
It mustn't be true.
He must be tired. -> He ___ tired.
A: Why is the store closed? B: It ___ a holiday.
Match 'must' to its meaning.
they / must / have / forgotten
I'm not sure, but it ___ rain.
Score: /8
FAQ (8)
No, 'must' for deduction is for present or past. Use 'might' or 'will' for the future.
Yes, 'may' is often used in formal writing, while 'might' is common in speech.
'Mustn't' is reserved for prohibition. Use 'can't' instead.
They are very similar, but 'could' sometimes implies a slightly higher possibility.
Yes, 'He must be working' is perfectly fine.
Yes, they are standard across all major English dialects.
Use 'couldn't have' or 'can't have'.
Rarely, but some literary forms use 'shall' for probability.
Scaffolded Practice
1
2
3
4
Mastery Progress
Needs Practice
Improving
Strong
Mastered
In Other Languages
Deber + infinitive
Spanish uses the conditional tense for probability, which English doesn't.
Devoir
French uses the conditional for uncertainty.
Modalverben
German word order changes with modals.
Kamoshirenai
Japanese does not use auxiliary verbs in the same way.
Yajib an
Arabic lacks a direct modal system like English.
Keneng
Chinese verbs do not conjugate for modals.
Learning Path
Prerequisites
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