Must and Can't (Dedcutions)
Intermediate (B1)en
Must/can't deduction
Modals for Deductions — Must and Can’t
Sometimes we make guesses about situations based on evidence or facts. This is called a deduction. Modals like must and can’t help us express how certain we are about these guesses.
1. What a Deduction Is
A deduction is a logical conclusion you make when you look at the evidence and decide what is likely or unlikely.
Example:
- “The lights are on. Someone must be home.” → Evidence: lights are on → deduction: someone is probably home
- “There are wet footprints in the hallway. It must have rained.” → Evidence: wet footprints → deduction: it rained
- “The kitchen is empty. They can’t be cooking now.” → Evidence: kitchen empty → deduction: impossible
Note: Deduction is about using clues to make a logical guess — not guessing randomly.
2. Using Must for Strong Deductions
Use must when you are almost certain something is true because of the evidence you see.
Examples:
- “She must be tired. She worked all night.”
- “He must live nearby. He walks to work every day.”
3. Using Can’t for Strong Negative Deductions
Use can’t when you are almost certain that something is impossible based on the evidence.
Examples:
- “He can’t be at home. His car isn’t here.”
- “That can’t be true. The shop is closed.”
- “She can’t have finished the project already. She started it yesterday.”
- “It can’t be snowing in July. That doesn’t happen here.”
4. Key Takeaway
- Must → strong logical certainty, very likely to be true
- Can’t → strong logical impossibility, very unlikely or impossible
- Deductions are based on evidence, not random guessing
Related Vocabulary
Expand your vocabulary with words related to this grammar topic