Must and Can't (Deductions)

Sometimes we make guesses about situations based on evidence or facts. This is called a deduction. A deduction is a “smart guess” based on clues (evidence). 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.

Examples

• “The lights are on. Someone must be home.” → Evidence: lights are on → deduction: someone is probably home
• “The kitchen is cold, and the stove is off. They can’t be cooking now.” → Evidence: kitchen cold + stove off → deduction: it’s impossible that they are cooking now

Note: Deduction is about using clues to make a logical guess — not guessing randomly.

2. Using must for strong deductions

Use must when the evidence makes you feel very sure something is true (a strong logical conclusion). If you are not sure, use might/may/could (not covered here).

Note: Here, must is not an obligation; it means ‘I’m sure this is true.’

Form:

• Now (state/general): must + verb (infinitive without ‘to’) (must be / must know)
• Now (in progress): must be + -ing (must be sleeping / must be eating)
• Past: must have + past participle (must have forgotten / must have slept)

Examples:

• “She must be tired. She worked all night.”
• “It’s so quiet in the house. Everybody must be sleeping.”
• “He must have been in the sun. He is sunburned.”

3. Using can’t for strong negative deductions

Use can’t when the evidence makes you sure it’s impossible / not true.

Form:

• Now (state/general): can’t + base verb (can’t be / can’t live)
• Now (in progress): can’t be + -ing (can’t be snowing / can’t be working)
• Past: can’t have + past participle (can’t have finished / can’t have gone)

Here, can’t means “it’s impossible (I’m sure)”.

Examples:

• “He can’t be at home. His car isn’t here.”
• “He can’t be working right now. He’s in the café with his children.”
• “She can’t have finished the project already. She started it yesterday.”

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