What are Modal Verbs of Probability: Might, May & Must?
Modal verbs of probability express how confident the speaker is that something is true. Instead of stating a fact directly, the speaker uses a modal to signal how much evidence they have and how certain their conclusion is. This is the language of reasoning, deduction, and speculation — essential for sounding natural and nuanced in English.
The key modals in this group are must, can't / couldn't, may, might, and could. Each sits at a different point on the certainty scale: must (almost certain — strong evidence supports the conclusion), can't / couldn't (almost certainly not — evidence rules it out), may (possible — a realistic option), might / could (possible but uncertain — more speculative).
The same modals that express probability in the present can also be used with have + past participle to make deductions about the past — one of the most important B2-level patterns in English modal grammar.
How to Form Probability Modals
All probability modals follow the standard modal pattern: modal verb + base form, no inflection. For past deduction, insert have between the modal and the past participle. The form is the same for every subject.
Present: modal + be / base verb · Past: modal + have + past participleProbability modals — present and past
| Certainty | Present deduction | Past deduction |
|---|---|---|
| Almost certain (+) | She must be tired. | She must have been tired. |
| Almost certain (−) | That can't be right. | That can't have been right. |
| Possible (~50%) | He may / might be at home. | He may / might have been at home. |
| Speculative | It could be a virus. | It could have been a virus. |
| Almost certain (−) past | — | They couldn't have known. |
When to Use Modal Verbs of Probability: Might, May & Must
Strong positive deduction — must
We use must when we are almost certain something is true based on evidence, logic, or reasoning. The speaker is not guessing — they have enough information to feel very confident. Must for deduction always describes a conclusion being drawn in the moment, not a rule or command.
- You've been travelling for 20 hours — you must be exhausted.
- She must know this area well — she grew up here.
- That must be Dr Patel — I recognise her from the conference photo.
Strong negative deduction — can't / couldn't
When the evidence points firmly to something being untrue or impossible, we use can't (present) or couldn't (past or more tentative). This is the logical negative of must for deduction. Note: mustn't is NOT used for negative deduction — can't is the correct form.
- That can't be the right address — there's no building there.
- She can't have received my email — she would have replied by now.
- You can't be serious — you're joking, right?
Present possibility — may / might / could
When something is possible but not certain — the speaker has some reason to believe it but doesn't have enough evidence to commit to must — we use may, might, or could. May is slightly more confident than might; could is often the most tentative, implying a speculative option. In practice, the three are frequently interchangeable.
- I might be late — there's a lot of traffic on the motorway.
- She may know the answer — try asking her before you give up.
- It could be a software issue, or it could be a hardware problem — hard to tell.
Past deduction — must have / can't have / might have
To make deductions about past events or states — drawing conclusions about what probably happened when you weren't there — we use modal + have + past participle. The same certainty scale applies: must have = almost certain (past), can't have = almost certainly didn't (past), might / may have = possibly (past).
- Her coat isn't here — she must have left already.
- They didn't answer at all — they can't have heard the doorbell.
- He might have gone home — his desk is clear and his computer is off.
Future possibility — may / might / could
The same modals that express present uncertainty also work for future possibility. When we are not sure whether something will happen, we use may, might, or could rather than making a definite prediction with will. This is especially common in weather forecasts, plans, and cautious predictions.
- It might rain later — you'd better take an umbrella.
- We may not arrive on time if the train is delayed.
- This could turn out to be one of the most important decisions you make.
Key Context Words and Phrases
Must for Deduction vs Must for Obligation
The word must does two completely different jobs in English. Both uses are common, but confusing them will lead to strange-sounding sentences. The key is to look at what the speaker is doing: are they drawing a conclusion, or issuing a requirement?
Must — deduction (reasoning from evidence)
You must be hungry — you haven't eaten since breakfast.
The speaker is drawing a logical conclusion from observable evidence. No command is being given. The meaning is "I'm almost certain you are hungry."
Must — obligation (imposing a requirement)
You must eat something before you go.
The speaker is imposing a requirement — not making a deduction. The meaning is "I require you to eat." This is a command, not a conclusion.
Must — deduction (about a third person)
He must know the area — he's never lost.
The speaker is making an inference: "I'm almost certain he knows." No obligation is placed on anyone.
Must — obligation (formal rule)
All visitors must sign in at reception.
A formal rule — signing in is required. The institution is imposing a necessity, not drawing a conclusion.
Can't vs Mustn't for Negation
When learners want to express that something is almost certainly not true, they sometimes reach for mustn't. This is wrong — mustn't is only used for prohibition. For deduction, English uses can't.
Can't — negative deduction (almost certainly not true)
That can't be her house — the lights are off and there's no car.
The speaker is drawing a conclusion: the evidence suggests it almost certainly isn't her house. This is deduction.
Mustn't — prohibition (it is not allowed)
You mustn't park here — it's a no-parking zone.
A rule about what is not permitted. Mustn't has nothing to do with deduction — it means "it is forbidden." Using mustn't for deduction is ungrammatical.
Common Mistakes
Using "mustn't" for negative deduction (use "can't" instead)
✗ That mustn't be the right answer — it doesn't make sense.
✓ That can't be the right answer — it doesn't make sense.
"Mustn't" expresses prohibition — something that is not allowed. To express near-certain negative deduction ("I'm almost sure this is not true"), always use "can't" or "couldn't". This is one of the most important distinctions in B2 modal grammar and is tested frequently in Cambridge exams.
Omitting "have" in past deductions
✗ She must left early — the coffee is cold. / They might forgot about it.
✓ She must have left early — the coffee is cold. / They might have forgotten about it.
Past deductions always require "modal + have + past participle". The word "have" cannot be omitted — it is what signals that you are talking about the past. "Must left" and "might forgot" are ungrammatical. Think of it as a three-part pattern: modal → have → past participle, every time.
Using "can be" (general possibility) instead of "could be" (specific speculation)
✗ I'm not sure where she is — she can be at the library.
✓ I'm not sure where she is — she could be / might be at the library.
"Can be" expresses general, theoretical possibility — that something is possible in principle (e.g. "the journey can be difficult in winter"). It does not express speculation about a specific current situation. When you are guessing or deducing about a particular situation right now, use "could be", "might be", or "may be".
Using "might" when strong evidence calls for "must"
✗ You've been awake for 30 hours — you might be tired. (said with certainty)
✓ You've been awake for 30 hours — you must be exhausted.
When the evidence is very strong, using "might" sounds oddly non-committal — as if the speaker genuinely has no idea. Match your modal to the strength of your evidence: strong evidence → must; genuine uncertainty → might / may.
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