Modal Verbs of Deduction

B2 · Upper IntermediateModal Verbs

Must and can't let you reason your way to a confident conclusion from the evidence in front of you — 'it has to be true' or 'it has to be false.'

What you'll learn

  • Must + base verb for a confident, logical positive conclusion
  • Can't + base verb for a confident, logical negative conclusion
  • Must have / can't have + past participle for deductions about the past
  • Can't, not mustn't, is used for a confident negative deduction

Reasoning to a confident conclusion

Must and can't let you draw a confident conclusion from evidence — not a guess, but something you're almost certain must be true or must be false.

"The lights are off, so they must be asleep."
"He can't be at work — I just saw his car at home."

Must (positive) vs. can't (negative)

Must draws a confident positive conclusion: it has to be true. Can't draws a confident negative conclusion: it has to be false. Note that can't, not mustn't, is the negative here — mustn't is reserved for prohibition, a completely different meaning.

"This must be the wrong address."
"That can't be right."

Deduction about the past

To draw a confident conclusion about something in the past, use must have or can't have + the past participle.

"She must have forgotten her keys."
"They can't have left yet — their car is still here."

Real English

"You must be joking."
"He must have missed the train."
"That can't have been easy."
"They can't have forgotten — I just reminded them."

Watch out

✗He mustn't be at work — I just saw his car at home.
✓He can't be at work — I just saw his car at home.

For a confident negative conclusion, use can't, not mustn't — mustn't means forbidden, a different idea entirely.

✗She must forgot her keys.
✓She must have forgotten her keys.

For a deduction about the past, use must have + past participle, not the simple past.

✗This must to be the wrong address.
✓This must be the wrong address.

Don't add to after must — it's followed directly by the base form.

Real life — Figuring out what happened

"The door's unlocked but no one's answering."

"They must have just stepped out for a minute."

"Maybe. Their bags are still here, though."

"True — they can't have gone far, then."

Practice

Exercise 1 · Choose the correct sentence

Choose the correct sentence — a confident positive conclusion from evidence.

Exercise 2 · Choose the correct sentence

Choose the correct sentence — a confident negative conclusion.

Exercise 3 · Choose the correct sentence

Choose the correct sentence — deduction about the past.

Exercise 4 · Choose the correct sentence

Choose the correct sentence — confident negative deduction about the past.

Exercise 5 · Choose the correct answer

Which modal signals a confident, logical certainty (positive conclusion)?

Exercise 6 · Choose the correct answer

Which modal signals a confident, logical certainty (negative conclusion)?

Exercise 7 · Choose the correct sentence

Choose the correct sentence.

Exercise 8 · Choose the correct sentence

Choose the correct sentence — deduction about the past, negative.

Exercise 9 · Complete the sentence

Complete: 'You ___ (must/be) exhausted after that flight.'

Exercise 10 · Complete the sentence

Complete (negative deduction, past): 'She ___ (can't/have/known) about the surprise.'

Exercise 11 · Sort into groups

must be
can't be
must have known
can't have known
must be tired
can't be true

0 / 6 sorted

Quick challenge

Five quick questions to check what stuck.

Question 1 / 5

Score: 0/0

Choose the correct sentence.

You learned

  • Must draws a confident positive conclusion from evidence.
  • Can't draws a confident negative conclusion from evidence.
  • Must have / can't have + past participle deduces something about the past.
  • Can't, not mustn't, is the negative form used for deduction.

Related grammar lessons

Upper IntermediateModal Verbs

Modal Perfects

Learn how should have, could have, and would have look back at the past to express regret, unrealized ability, and hypothetical outcomes.

Upper IntermediateModal Verbs

Modal Verbs of Possibility

Learn how to express possibility in English with may, might, and could — for the present, future, and past.

IntermediateModal Verbs

Need, Needn't and Don't Need To

Learn the two faces of need in English — as a modal verb (needn't) and as a regular verb (don't need to) — and how needn't have + past participle works.

IntermediateModal Verbs

Should, Ought To and Had Better

Compare three ways to give advice — should, ought to, and had better — and learn how their grammar and strength differ.

View all lessons in this category