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'Must' can express obligation or deduction; 'can't' (not 'mustn't') expresses the negative deduction.
Side-by-side comparison
"Must" in deductions expresses positive certainty: you are almost sure something is true based on the available evidence.
She passed every single exam without studying, she must be incredibly smart.
They have not eaten all day, so they must be starving by now.
That cannot be his sister, they look absolutely nothing alike.
"Can't" in deductions expresses negative certainty: you are almost sure something is impossible or cannot be true. It is the logical opposite of "must".
You must always wear a helmet when you ride a bike, it is the law.
Students must not use their phones during the exam under any circumstances.
You must try this restaurant, the food is absolutely incredible.
Common mistake
"He mustn't be here yet" is wrong for negative deduction: use "can't". "He can't be here yet" = it is impossible that he has already arrived. "Must" is for positive deduction. "Can't" is for negative deduction.
Trick to remember it
- •Positive deduction (logically certain to be true): "must".
- •Negative deduction (logically impossible): "can't".
- •Never "mustn't" for deductions.
- •Past: must/can't + have + past participle.
Quick rule
- •Positive deduction: "must be" (it must be).
- •Negative deduction: "can't be" (it cannot be).
- •"Mustn't" = prohibition, not deduction.
Did you know?
The epistemic MUST (deduction) and the deontic MUST (obligation) are the same grammatical form with two completely different meanings, and only context allows you to distinguish them. In English, the same form "she must go" can be obligation or deduction. Detective Sherlock Holmes is famous for constantly using the epistemic MUST: "You must have been in Afghanistan, I perceive", he is not saying Watson has an obligation to have been in Afghanistan; he is deducing it is logically impossible that he was not.
Practise what you learned
Exercise 1 · Pick the right one
She answered every question correctly. She _____ have studied a lot.
Exercise 2 · Pick the right one
He _____ be tired. He slept for twelve hours and just woke up.
Exercise 3 · Form the word
Related word pairs
'Have to' expresses external obligation; 'need to' expresses functional or personal necessity.
"Must" is internal obligation or logical deduction; "have to" is external obligation imposed by others.
'Must' indicates obligation (you have to do it); 'mustn't' indicates prohibition (you must not do it).
'Should have' expresses a past obligation not fulfilled; 'could have' expresses past capacity or possibility not acted upon.
Bryan López
Founder of SpeakUP Academy