Possibility & Deduction (may/might/must)

Modal verbs can show how certain you are about something — from a faint "maybe" all the way to an almost-sure "definitely". May, might, could and must form a little scale of certainty, and learning to slide up and down it is a genuinely useful skill. Why does this confuse students? Mainly because must is the troublemaker: you've probably learned it as a rule word ("you must tidy up"), and now here it is meaning "I'm sure". Same word, different job. Once you can tell those two musts apart, this whole topic falls neatly into place.

The certainty scale

• might / may / could → possible, but you're not sure: It might rain. She may be at home. That could be true.
• must → almost certain, based on evidence: The ground's wet — it must have rained.
• can't → almost certainly not: He can't be hungry; he's just eaten.

So the scale runs from a shaky "maybe" up to a confident "surely": might / may / could → must, with can't at the far "no way" end. Picture a dial in your head, and you'll always know roughly where each word sits.

Deduction: working it out from the clues

Must and can't are for deductions — conclusions you reach by looking at the evidence in front of you. You're not guessing wildly; you're being a detective:

The lights are off, so they must be out. (the evidence points straight to it) She can't be forty — she looks twenty-five! (the evidence rules it out)

The key is that a deduction needs clues. If you've got a good reason — an empty drive, a wet umbrella, a half-eaten sandwich — you've earned your must or can't. If you've only got a vague hunch, drop back down the scale to might or could.

A second angle: present versus past

The scale works for the past too — you just change the shape slightly. For the present, you use the plain verb: "He must be at lunch." For the past, you add have + the past-participle: "He must have left already."

The pitch is soaked — it must have rained overnight. (a sure guess about the past) She might have missed the bus. (a weaker guess about the past)

One thing to watch: it's always must have, never must of. That mistake sneaks in because have and of sound the same when we speak. In writing, always have.

Watch out — this "must" isn't a rule

Here's the big one. Must has two completely different jobs, and only the sentence tells you which:

You must be tired. → I bet you're tired (deduction — I'm sure) You must tidy your room. → it's a rule (obligation — you have to)

The first must is reading the evidence — you've just run a race, so of course you're tired. The second must is laying down a rule. Same word, opposite jobs. When you meet a must, ask yourself: is this "I'm sure" or "you have to"? The rest of the sentence always gives it away.

More worked examples

• ✓ Her coat's gone, so she must have left. (evidence-based deduction about the past)
• ✓ It might snow tonight — the sky looks heavy. (a real possibility, not certain)
• ✗ He must of forgotten. → ✓ He must have forgotten. (never "must of")
• ✓ That can't be Sam — he's away on holiday. (the evidence rules it out)
• ✗ She must be the new girl (with no clues at all) → weaker but honest: ✓ She might be the new girl. (no evidence yet, so don't over-claim)

An edge case: saying "no" needs "can't", not "mustn't"

Here's a neat little rule that surprises people. When you're sure something is true, you use must: "He must be home." But when you're sure something isn't true, you don't flip it to mustn't — you reach for can't instead: "He can't be home; his car's gone." Try saying "He mustn't be home" and your ear will tell you it sounds off, because mustn't has drifted off to mean "it's not allowed" (a rule) rather than "it's not possible" (a deduction). So the pair to lock in is this: must for a sure yes, can't for a sure no. "That must be the answer" and "That can't be the answer" are the two ends of the same deduction, and mixing in mustn't by mistake will quietly muddle your meaning.

Why it matters

These modals are how you sound careful rather than cocky — and that's exactly what markers reward. In a science answer, writing "these results might suggest a link" is far wiser than a flat "this proves it", because it shows you understand that evidence can be partial. In everyday life, the scale keeps you fair: "You must have taken my rubber" sounds like an accusation, while "Have you seen my rubber? You might have picked it up by mistake" keeps the peace. Being able to dial your certainty up and down — matching how strong your language is to how strong your evidence is — is one of the most grown-up moves in writing. It tells your reader you know the difference between what you know and what you're only guessing.

Quick check

Choose the best modal:

1. (a guess, not sure) She ___ be at the library.

2. (sure, from evidence) His car's gone, so he ___ have left.

3. (ruling it out) That ___ be Mr Khan — he's off sick today.

4. (spot the error) Fix: "They must of finished by now."

5. (which "must"?) Is "You must be freezing in that thin coat" a rule or a guess?

Where next

• Ability and Permission — can/could/may for able and allowed.
• Obligation and Advice — must meaning "have to".
• Modal Verbs Overview — the whole family.