What are modals of possibility in English?

They are the modals that grade how sure you are about a present situation: may, might, and could for a chance, must for a confident deduction, and can't for a confident 'that is impossible'. The signal might be interference is a guess. Yasmin must be in the control room is a conclusion from evidence. That can't be the pulsar is a rejection of a reading.

What is the difference between must be and have to?

Must be on this page is deduction: I conclude that this is true (She must be in the control room). Have to is obligation: a rule or a duty (She has to log the pointing). You must be quiet can be a rule; You must be Yasmin is a deduction. If you can add 'I conclude from the evidence', you are in possibility, not obligation.

Is maybe a modal verb?

No: maybe (and perhaps) is an adverb that sits at the front or in mid position and leaves the verb alone (Maybe the feed horn is iced up). A modal occupies the verb slot instead (The feed horn might be iced up). Pick one system; Maybe might be iced up stacks them and is the error to avoid.

Why is the negative of must be not mustn't?

Because mustn't means 'it is forbidden' (You mustn't park the dish without a log). To say a present reading is impossible, English uses can't: That can't be the pulsar. Might not and may not mean 'it is possible that this is not so', which is weaker than can't.

How is this different from may vs. might vs. could?

The three-word page compares those three forms with each other: register, remoteness, and permission. This page places them on a wider present-tense scale that also includes must and can't of deduction, and it contrasts the whole modal system with maybe. Past guesses (must have, might have) belong with past modals.

English grammar · Intermediate

Modals of Possibility: May, Might, Could, Must, and Can't

Written by Eleanor Whitfield, Lead English Curriculum Editor · Reviewed September 2026

Modals of possibility grade how sure the speaker is about a present situation: may, might, and could for a real chance, must for a confident deduction, and can't (or couldn't) for a confident 'that is impossible'. They occupy the finite slot (It might be interference; That can't be the pulsar). Maybe and perhaps are adverbs, not modals: they comment on the clause without changing the verb. This page is present deduction and present chance, not the three-word comparison of may, might, and could, and not past forms like must have.

The radio dish is tracking; Yasmin must be in the control room.

must plus be: a confident deduction about now, not a rule

What it is

A modal of possibility does not narrate a fact. It reports the speaker's reading of the present. Might, may, and could say there is a chance (The signal might be interference). Must says the evidence points one way (She must be in the control room: the dish is tracking). Can't says the evidence rules a reading out (That can't be the pulsar: the frequency is wrong). The scale is the whole lesson. Maybe it is interference leaves the verb in the ordinary present and puts the uncertainty in an adverb. It might be interference puts the uncertainty in the verb slot itself. Both are possible; they are not the same grammar.

The dish is locked on a source; they must be calibrating. (confident deduction) The dish is locked; they might be calibrating. (one possible reading) That can't be calibration; the tone is wrong. (confident rejection)

How to spot it

Look for may, might, could, must, or can't plus a bare verb, most often be: might be, must be, can't be. If the sentence is a guess about now, you are on this page. If it is a guess about a finished event (must have missed, might have broken), you want past modals. If the uncertainty word is maybe or perhaps, you want adverbs of possibility.

The signal might be interference from the weather radar. (might: a real present chance)
Yasmin must be in the control room; the dish is tracking. (must: a deduction from evidence)
That can't be the pulsar; the period is wrong. (can't: the evidence makes it impossible)
Maybe the feed horn is iced up. (maybe: an adverb, the verb stays ordinary)

Could here is not past ability. In a present guess, could is a weaker may: The delay could be a software glitch. Could I leave the dish parked? is permission, a different job.

Modals of Possibility Quick Reference

Present possibility and deduction on one scale

Form	Certainty	Example
must + be / verb	confident deduction: the evidence points this way	She must be in the control room.
may + be / verb	a real possibility, slightly formal	The signal may be interference.
might + be / verb	a possibility, often a little remoter than may	The feed horn might be iced up.
could + be / verb	a possible explanation among others	The delay could be a software glitch.
can't / couldn't + be	confident negative deduction: impossible on this evidence	That can't be the pulsar.
maybe / perhaps + ordinary verb	adverb of possibility, not a modal	Maybe the feed horn is iced up.

Common Modals of Possibility Examples in English

Grouped by how sure the speaker is, which is the only scale these present modals sit on:

A Real Chance (May, Might, Could)

The signal might be interference from the weather radar.: might: one possible reading of now
The dish may still be parked on the old source.: may: a real chance, a little more formal
The delay could be a software glitch in the correlator.: could: one explanation among others
Yasmin might not have the overnight notes with her.: might not: a negative possibility, not a deduction

May, might, and could overlap heavily for a present chance. Might is often a shade more tentative than may. Could offers an explanation rather than a forecast.

Confident Deduction (Must)

Yasmin must be in the control room; the dish is tracking.: must be: evidence, then a conclusion
They must know the period already; the plot is labelled.: must plus a main verb, still a deduction
The feed horn must be clear: the noise floor has dropped.: must be plus an adjective
This must be the calibration tone; it repeats every four seconds.: must be: identifying something in front of you

Must of possibility is not must of obligation. You must be in the control room (I conclude that you are) is not You must sit in the control room (I require it).

Confident Rejection (Can't)

That can't be the pulsar; the period is wrong.: can't: the evidence rules it out
Yasmin can't be on the access road; her badge is on the hook.: can't be: a present impossibility
This couldn't be the overnight plot; the date stamp is Tuesday.: couldn't: a slightly softer can't
They can't still be calibrating; the dish has been parked for an hour.: can't plus a continuous: not possible now

For a present deduction, the closed-off reading is can't, not must not. Must not is a ban. Can't of possibility says the evidence makes that reading impossible.

Maybe vs. a Modal

Maybe the feed horn is iced up.: maybe plus an ordinary present
The feed horn might be iced up.: the same guess, uncertainty in the verb
Perhaps they are still in the control room.: perhaps at the front, verb unchanged
They could still be in the control room.: could plus be: modal version of the same idea

Maybe and perhaps never change the verb. May, might, and could replace the ordinary present with a modal plus a bare verb. You cannot stack them as maybe might be in careful English; pick one system.

How Modals of Possibility Work

One scale of certainty, one grammar, and a negative that does not match the affirmative.

The Modal Goes in the Finite Slot

May, might, could, must, and can't are followed by a bare verb, most often be: might be iced up, must be in the control room, can't be the pulsar. There is no -s, no to, and no do. Questions invert the modal (Could that be interference?). Negatives sit on the modal (might not, may not, can't). Maybe cannot fill this slot: Maybe be interference is impossible.

Could that be interference? It might not be the pulsar.

modal inverts for the question and takes the negative

Modal + bare verb, usually be.

Must Deduce; May, Might, and Could Guess

Must of possibility is a conclusion from evidence, not a rule. You only use it when something in the situation supports the claim: the dish is tracking, so she must be in the control room. May, might, and could do not need that support; they only need a chance. Using must for a mere chance (She must be here, I haven't checked) overstates the evidence.

The dish is tracking; she must be in the control room. The dish is idle; she might be on the access road.

must needs evidence; might only needs a chance

Must = I conclude. Might = there is a chance.

The Negative of Deductive Must Is Can't

Mustn't means 'you are forbidden'. It is not the negative of must be. To reject a present reading with confidence, English uses can't (or couldn't): That can't be the pulsar. May not and might not are weaker: they leave the possibility open (It might not be interference, but it could be). The scale in the negative is can't at the top and might not / may not in the middle. There is no everyday mustn't be of deduction.

That can't be the pulsar. It might not be interference.

can't rules it out; might not leaves a chance it isn't

Deduction negative = can't, not mustn't.

Maybe Is an Adverb, Not a Modal

Maybe and perhaps sit outside the verb phrase and leave the verb in whatever tense the facts need: Maybe the feed horn is iced up. Perhaps they are still calibrating. A modal replaces the present: The feed horn might be iced up. Both grade possibility. Only the modal occupies the auxiliary slot, builds the question (Might it be iced up?), and combines with be -ing (They might be calibrating).

Maybe the feed horn is iced up. The feed horn might be iced up.

same guess: adverb plus ordinary verb, or modal plus bare verb

Maybe leaves the verb alone. Might replaces it.

Common Mistakes with Modals of Possibility

Incorrect: Yasmin must to be in the control room.

Correct: Yasmin must be in the control room.

After a deduction must, the next verb has no to: must be, must know. Must to be copies have to, which belongs to obligation, not to this reading of evidence.

Incorrect: That mustn't be the pulsar; the period is wrong.

Correct: That can't be the pulsar; the period is wrong.

In deduction, the closed-off reading is can't (or couldn't), not must not. Must not is a prohibition. The evidence rules a guess out; it does not issue a ban.

Incorrect: Maybe the feed horn might be iced up.

Correct: Maybe the feed horn is iced up. / The feed horn might be iced up.

Maybe and might both grade possibility. Stacking them is redundant and clumsy. Pick the adverb system (maybe + ordinary verb) or the modal system (might + bare verb).

Incorrect: They must have been calibrating now; the dish is nodding.

Correct: They must be calibrating; the dish is nodding.

Must have plus a participle looks back at the past. A deduction about an action in progress now is must be plus -ing. Keep have for past-modals territory.

Must vs. Can't, and Continuous Deductions

The two ends of the scale, and the be -ing form that locates the guess in the present moment.

Must and can't as a pair

Must says 'on this evidence, this is the reading'. Can't says 'on this evidence, this reading is impossible'. They share a job (deduction) and split on yes vs. no.

The dish is tracking; she must be in the control room.: must: the evidence supports it
Her badge is on the hook; she can't be on the access road.: can't: the evidence rules it out
This must be the calibration tone. That can't be the pulsar.: the two ends of the same scale

Mustn't does not belong in this pair. It forbids an action (You mustn't park the dish without a log entry).

Modal plus be plus -ing

To guess about an action in progress now, use modal + be + -ing: they must be calibrating, she might be driving up the access road, they can't still be aligning the feed.

They must be calibrating; the dish is nodding.: must be + -ing: deduction about now
She might be checking the feed horn on the platform.: might be + -ing: a possible present action
They can't still be aligning it; the lock light is on.: can't be + -ing: not possible as a present action

Modal + have + past participle (must have calibrated) is a past deduction and belongs on the past-modals page.

May not vs. can't

May not and might not mean 'it is possible that this is not the case'. Can't means 'it is not possible that this is the case'. The not sits in different places on the scale.

It might not be interference.: a chance that it isn't
It can't be interference; the weather radar is off.: no chance that it is
She may not have the overnight notes with her.: possible that she doesn't

This is why learners who want a polite 'perhaps not' should not reach for can't, and why learners who want 'impossible' should not reach for might not.

Quick Check: Modals of Possibility

Choose may/might/could, must, can't, or rewrite with maybe.

1. The dish is tracking; Yasmin ___ be in the control room. (confident deduction)

Answer: must

evidence, so must, not might

2. That ___ be the pulsar; the period is wrong. (impossible)

Answer: can't / couldn't

negative deduction is can't, not mustn't

3. The delay ___ be a software glitch. (one possible explanation)

Answer: could / might / may

a chance, not a conclusion

4. ___ the feed horn is iced up. (adverb, not a modal)

Answer: Maybe / Perhaps

adverb plus the ordinary present is

5. They ___ calibrating; the dish is nodding. (deduction about now)

Answer: must be

must be plus -ing, not must have

Modals of Possibility FAQs

What are modals of possibility in English?: They are the modals that grade how sure you are about a present situation: may, might, and could for a chance, must for a confident deduction, and can't for a confident 'that is impossible'. The signal might be interference is a guess. Yasmin must be in the control room is a conclusion from evidence. That can't be the pulsar is a rejection of a reading.
What is the difference between must be and have to?: Must be on this page is deduction: I conclude that this is true (She must be in the control room). Have to is obligation: a rule or a duty (She has to log the pointing). You must be quiet can be a rule; You must be Yasmin is a deduction. If you can add 'I conclude from the evidence', you are in possibility, not obligation.
Is maybe a modal verb?: No: maybe (and perhaps) is an adverb that sits at the front or in mid position and leaves the verb alone (Maybe the feed horn is iced up). A modal occupies the verb slot instead (The feed horn might be iced up). Pick one system; Maybe might be iced up stacks them and is the error to avoid.
Why is the negative of must be not mustn't?: Because mustn't means 'it is forbidden' (You mustn't park the dish without a log). To say a present reading is impossible, English uses can't: That can't be the pulsar. Might not and may not mean 'it is possible that this is not so', which is weaker than can't.
How is this different from may vs. might vs. could?: The three-word page compares those three forms with each other: register, remoteness, and permission. This page places them on a wider present-tense scale that also includes must and can't of deduction, and it contrasts the whole modal system with maybe. Past guesses (must have, might have) belong with past modals.