Grammar · Modal Verbs
Modals of Deduction (Must, Can’t, Might)
Must, can’t and might show how sure you are. Look at the clues and make a good guess, with the film The Adventures of Tintin.
Grammar › Modal Verbs › Modals of Deduction (Must, Can’t, Might)
The rule in one sentenceWhen the clues make you almost sure something is true, use must; when they make you almost sure it is false, use can’t; when you really don’t know, use might.
About the film
The Adventures of Tintin is a film from 2011 about Tintin, a young reporter who lives with his clever white dog, Snowy. At a street market, he buys a model of an old ship called the Unicorn. Right away, two different men try to buy it from him, so Tintin thinks it can’t be just a toy – it must have a secret.
What does it do?
These words show how sure you are. You don’t know the answer for certain, but you look at the clues and make a guess. Tintin does this all the time – it’s his job!
Use must when the clues make you almost sure something is true. Use can’t when the clues make you almost sure it is false. When you are not sure at all, use might, may or could.
Look at each tile. The clue comes first, then the guess. Notice that the opposite of must here is can’t, not mustn’t. Mustn’t is for rules, not for guesses.
In speaking, people almost always use the short forms. But be careful with mustn’t:
The man pays a lot for an old toy ship. It must be special. Is it just a cheap toy? No, that can’t be true.
First look at the clue. Then choose how sure you are.
Visual guide
Teacher Teddy’s illustrated guide to this rule is being drawn — the full swipe-through infographic will appear here soon.
How to make it
must — I’m sure it’s true
Sakharine must know the secret of the Unicorn.
That must not be easy for Tintin.
Rare. For a strong “no”, most people use can’t.Is the model special? Yes, it must be.
Questions with must are rare. Ask “Do you think…?” instead.can’t — I’m sure it’s false
The thief can’t be far away.
Can’t already means “no”, so the rest of the sentence stays positive.The paper cannot be old rubbish.
Cannot is the full form. It is more common in writing.Could it be a map?
In questions, use can or could: Could it be…? Can it be true?When to use each one
must — the clues say yes
can’t — the clues say no
Words that help: yes or no?
These words often come just before the guess. They show which way the clues point.
must → I’m sure it’s true
can’t → I’m sure it’s false
Common mistakes
❌ The model mustn’t be a normal toy.
✅ The model can’t be a normal toy.
For a strong “no” guess, use can’t. Mustn’t is for rules.
❌ The ship must to be special.
✅ The ship must be special.
Never put “to” after must.
❌ Maybe the paper must be a map.
✅ The paper might be a map.
Maybe means you aren’t sure, but must means you are. Use might.
❌ The thief can’t is far away.
✅ The thief can’t be far away.
After can’t and must, use be, not is.
One more thing
This part is a little harder. You can come back to it later.
Guessing about the past uses the same words with have + past participle. The model has gone from Tintin’s flat, so he makes guesses about what happened earlier.
✅ Someone must have taken the model. (I’m sure it happened)
✅ It can’t have been Snowy. (I’m sure it didn’t)
There is a full lesson on this: Modal Verbs of Speculation.
Practice: how sure are you?
Eight questions. Look at the clue, then choose. You get the answer, and why.
1. Two men want the model ship. It ___ be special.
Two men want it – a strong clue for yes → must.
2. Is the model just a cheap toy? That ___ be true.
The clues say no → can’t.
3. We have no clues yet. The paper ___ be a map – or it might not.
No clues, so you aren’t sure → might.
4. Tintin heard a noise a minute ago. The thief ___ be far away.
Only a minute ago, so it’s almost impossible → can’t.
5. Snowy is barking at the door. Someone ___ be outside.
The barking is a strong clue for yes → must.
6. The ship must ___ special.
No “to” after must.
7. That ___ be Snowy. Snowy is white, and this dog is black.
A sure “no” guess → can’t. Mustn’t is for rules.
8. Haddock has had a lot to drink. He ___ be feeling well.
The clue says he is not well → can’t.
Your turn
Choose one ending. Then listen to your sentence.
Every sentence here is correct. Pick the one you like best. Say it out loud too!
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