Step 55: It Must Be True
Logical conclusions — Must / can't for deduction
📖 Read — Detective Ethan
Ethan loves mysteries. This morning, he is playing detective at home. The kitchen light is on, and music is playing. “Someone must be in the kitchen,” he thinks. “It can't be Dad — Dad left for work at seven. It must be Mom!”
At school, more clues! Lily is smiling and holding flowers. “She must be happy,” says Ethan. Tom is yawning. “He can't be hungry — he ate two sandwiches! He must be tired.” Ethan laughs. “Detective work is easy when you read the clues!”
🧩 Grammar — Must and Can't — Logical Deduction
When the CLUES make you sure, use must for 'I'm sure it's true': The light is on — someone must be home. Use can't for 'I'm sure it's NOT true': He can't be hungry — he just ate! The opposite of deduction-must is can't, NOT 'mustn't' (mustn't = not allowed). In the middle (not sure)? Use may/might from Step 54.
- She must be tired — she ran ten miles.
- He can't be at school — it's Sunday.
- That must be Tom's bike — it's red and old.
- You can't be serious!
🏗️ Key structures
- He must be ___ (clue!) — He must be rich — look at his car!
- She can't be ___ (clue!) — She can't be cold — she's wearing a T-shirt in the snow… wait, she must be cold!
- It must ___ / It can't ___ — It must be the wind. / It can't be Tom — he's in Boston.
⭐ Idiom & natural phrases
Her email was short — but reading between the lines, I knew she was sad.
🔤 New words
a small piece of information that helps you solve a mystery
The wet shoes are a clue — someone was outside.
a person whose job is to find out what happened
The detective looked for clues.
something strange that you don't understand yet
Where he went is a mystery.
unusual; not normal; a little surprising
I heard a strange noise last night.
easy to see or understand; clear
It's obvious she's happy — look at her smile!
something that shows a thing is definitely true
Do you have proof?
why something happens; the cause
The reason he's late is the traffic.
something that only a few people know
It's a secret — don't tell anyone!
🎧 Say it with me
The light is on, so someone must be home. It can’t be Dad — he left at seven. It must be Mom!
🎤 Your turn to speak
Be a detective: look at three situations (a tired runner, an empty classroom, a smiling friend) and make one must-be and one can't-be deduction for each clue you invent.
Say it out loud three times. Even better: record it on your phone and listen — your ear is your best teacher.
✍️ Your turn to write
Write 4 deduction sentences about people you know: two with must be, two with can't be — add the clue after a dash.
See a model answer
My brother must be at the gym — his bag is gone. My teacher can't be sick — she's laughing with the class. The neighbors must be on vacation — their car is gone. And my cat can't be hungry — her plate is full!
Self-check: capital letters? Full stops? Did you use the grammar of this step?
📝 Quick check
Go deeper
This step is a doorway — the full lesson goes much further:
Modals of Deduction: must / can't / might have — full lesson + 50-question quiz
🎉 Saved in your progress!