🧭 Grammar · Modal Verbs
Modals of Deduction (Must, Can’t, Might)
Turn evidence into confident guesses about the present — the exact modal for “certainly true”, “certainly false” and “maybe”.
Grammar › Modal Verbs › Modals of Deduction (Must, Can’t, Might)
The rule in one sentenceUse must when the evidence makes something almost certainly true, can’t when the evidence makes it almost certainly false, and might / may / could when the evidence leaves real doubt either way.
What’s the difference?
Deduction modals aren’t about permission or ability — they’re about how confident you are in a guess, based on evidence you can see. English gives you a scale from certain to uncertain, and the modal you choose tells your listener exactly where on that scale you stand.
Must is your strongest “yes” — you’re not 100% sure, but the evidence points so clearly one way that any other explanation seems unlikely: The editing suite lights are still on at 2am — Tim Burton must be working late on the final cut of his next film.
Can’t is your strongest “no” — logic rules the idea out completely: That can’t be Peter Weller doing the stunt himself — reports confirmed he injured his ankle on the RoboCop set last month. Notice that English never uses “mustn’t” here — the negative of deduction-must is always can’t, not mustn’t.
🎬 Compare: “Julia Roberts’s trailer is parked outside the set, so she must be filming today” (strong positive evidence) vs. “That can’t be Ewan McGregor in the crowd — he’s currently promoting a film on the other side of the world” (strong evidence against it) vs. “Nobody has confirmed anything, so the rumoured Crash sequel might happen — or it might not” (genuinely unclear).
Same grammatical shape, three completely different confidence levels.
When the evidence is real but doesn’t point clearly one way, might, may and could all work and are largely interchangeable: Nobody has seen the final script, so the ending of the next season of Fargo might still be a secret even the cast doesn’t know.
How to build it
Must — the evidence says “certain”
Robert Downey Jr. must know the entire Marvel Cinematic Universe timeline after nine films as Tony Stark.
general state — certain factThe queue outside Hall H must be enormous right now — Marvel panels always sell out first at Comic-Con.
happening right now — must + be + -ing / must beCan’t — the evidence says “impossible”
That can’t be Daniel Radcliffe in the trailer — he stepped away from playing Harry Potter over a decade ago.
Danny Boyle can’t be shooting the Trainspotting sequel yet — the production is still finalising its cast.
action right now — can’t + be + -ingMight / May / Could — genuinely unclear
A Crash sequel might get announced at some point — nothing is confirmed yet.
Jeff Bridges could be filming reshoots for a new project this month, judging by recent set photos.
action right now — might/may/could + be + -ingWhen to use which
Use must / can’t when…
- The evidence is strong and specific: The set is empty and the crew has gone home — filming must have wrapped for the day.
- Logic rules an option out completely: That can’t be Christian Bale — he wasn’t cast in this project.
- You’d be genuinely surprised to be wrong.
Use might / may / could when…
- Evidence is mixed, rumoured, or partial: Reports say Andie MacDowell might return for a Four Weddings and a Funeral follow-up, but nothing is signed.
- You’re guessing without real proof: Scorsese could be developing another Scorsese-De Niro collaboration — it’s just speculation.
- You’d be mildly surprised either way.
Signal words
These are the phrases that usually trigger a deduction sentence in the first place.
Visual guide
Teacher Teddy’s illustrated guide to this rule is being drawn — the full swipe-through infographic will appear here soon.
Common mistakes
❌ Tom Hardy mustn’t be under the Bane mask — that’s clearly a stunt double.
✅ Tom Hardy can’t be under the Bane mask — that’s clearly a stunt double.
Mustn’t means prohibition (“you’re not allowed to”), never a confident “no” — that job belongs to can’t.
❌ He must to be filming in New Zealand right now.
✅ He must be filming in New Zealand right now.
Must is followed directly by the base verb or “be” — never add “to” after it.
❌ Maybe Winona Ryder must be in the new trailer.
✅ Winona Ryder might be in the new trailer.
Maybe signals uncertainty, and must signals certainty — the two contradict each other in one sentence.
❌ Must Scorsese be directing the new film?
✅ Do you think Scorsese is directing the new film?
Deduction-must almost never inverts into a question — English asks with “Do you think…?” instead.
Exceptions & tricky cases
Four cases where this rule connects to grammar you’ll meet elsewhere.
1. Deduction about the past uses a different pattern. Must/can’t/might + have + past participle covers guesses about something already finished — that’s its own lesson, but you’ll recognise it: Leonardo DiCaprio must have trained for months to survive the freezing water shots in The Revenant.
2. “Could” has two unrelated jobs. As deduction it means “maybe”: Millie Bobby Brown could be filming the Stranger Things finale right now. As a past-ability modal it means something completely different: As a child, she could already act confidently on camera. Context tells them apart.
3. “Should” sits nearby but isn’t deduction. Reviews for the new Paul Verhoeven film should be online by now expresses mild expectation, not evidence-based certainty — should belongs with advice and expectation, a different modal family.
4. Continuous vs simple form changes the meaning. Use the plain base verb for a general fact and “be + -ing” for something happening right now: Emma Thompson must know most of her lines from memory (general state) vs. She must be rehearsing right now (happening this moment).
Practice: how sure are you?
Eight quick questions. Pick the modal that matches the evidence — you get the explanation either way.
1. The lights are still on in the editing suite at 2am — Ang Lee ___ be working late on the final cut.
Strong, specific evidence (lights on at 2am) points clearly one way → must.
2. That ___ be Arnold Schwarzenegger doing the stunt himself — reports confirmed he injured his ankle last week.
Confirmed evidence rules the idea out completely → can’t, not mustn’t.
3. Nobody has confirmed anything, so the rumoured Crash sequel ___ happen — or it might not.
Unconfirmed rumour with no strong evidence either way → might.
4. Jennifer Grey’s trailer is parked outside the set — she ___ be filming today.
Visible, direct evidence (her trailer is there) → must.
5. The queue outside Hall H ___ be huge right now — Marvel panels always sell out first at Comic-Con.
A well-known, reliable pattern (Marvel panels always sell out) → must.
6. That ___ be Hugh Grant in the crowd — he’s currently promoting a film on the other side of the world.
Clear evidence he’s elsewhere rules it out completely → can’t.
7. Nobody’s seen the final script, so the ending of the new season of True Detective ___ be a secret even the cast doesn’t know.
No hard evidence, just a reasonable guess → might.
8. The director hasn’t replied to any interviews, so critics ___ be right about the reshoots — nobody knows for sure.
Silence isn’t strong proof either way — that keeps it at might, not must.
Keep going
Related lessons in Modal Verbs:
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