Lesson

Must, can't for deduction

Drawing conclusions

≈ 15 min 8 block(s)

Text

Must and Can't for deduction

We use 'must' and 'can't' to make logical deductions — conclusions based on evidence or knowledge. 'Must' expresses a strong certainty that something is true. 'Can't' expresses that something is very unlikely or impossible.

  • 'Must' + base verb for present deduction: She must be here.
  • 'Must have' + past participle for past deduction: He must have left.
  • 'Can't' + base verb for present impossibility: They can't be serious.
  • 'Can't have' + past participle for past impossibility: She can't have seen that.

These uses are epistemic (about certainty), not deontic (obligation).

Table

Must / Can't for deduction — quick reference

Form Use Example
must + base verb Present deduction (high certainty) She must be in the meeting.
must have + past participle Past deduction (high certainty) He must have finished the report.
can't + base verb Present impossibility They can't be at the office — the lights are off.
can't have + past participle Past impossibility She can't have received the package yet.

Tip

Key rule: how to form deductions

Form deductions using the modal + verb structure depending on time:

  • Present deduction (certainty now): must / can't + base verb → She must be online.
  • Past deduction (certainty about past): must have / can't have + past participle → They must have left.
  • Use evidence or knowledge (sight, facts, schedules) to support the deduction.

Remember: here 'must' is about certainty, not obligation.

Example

Examples in context

He must be in the office; his car is here.

They can't be at the meeting — the calendar shows them free.

She must have finished the report; she emailed it this morning.

He can't have received the package yet; tracking says 'in transit'.

Tip

Common mistakes to avoid

Watch out for these universal errors learners make with deduction modals:

  • Mixing obligation and deduction: 'must' can mean 'obliged to' or 'certain' — check the context.
  • Incorrect past form: forgetting 'have' in past deductions ('He must left' → wrong).
  • Using simple past after 'can't' for past impossibility ('She can't saw it' → wrong).
  • Overstating certainty: 'must' shows strong certainty, not absolute fact — consider softer options if unsure.
  • Wrong word order in questions or with negatives (keep modal before main verb).

When in doubt about time, check whether you need the perfect ('have' + past participle) for past conclusions.

Quiz

Choose the sentence that shows a deduction about the past:

Hint: Focus on verb form for past deductions.

Quiz

Complete: He _____ forgotten his laptop; his bag is empty.

Hint: Think about how to express certainty about the past.

Quiz

Which sentence indicates something is impossible now?

Hint: Check present vs past and modal 'can't' usage.

Key Points

GrammarPoint

must

modal verb CEFR B2 //mʌst//

A modal verb used to express strong certainty or obligation depending on context.

She must be at the client meeting by now.

GrammarPoint

can't

modal verb CEFR B2 //kænt//

A modal verb used to express logical impossibility or disbelief.

They can't be in the office; the lights are off.

GrammarPoint

must have

modal + perfect CEFR B2 //mʌst hæv//

Used for making a logical conclusion about a past event (must have + past participle).

He must have sent the invoice; I received it this morning.

GrammarPoint

can't have

modal + perfect CEFR B2 //kænt hæv//

Used to say something was impossible in the past (can't have + past participle).

She can't have finished already; she only started this morning.

Vocabulary

deduction

noun CEFR B2 //dɪˈdʌkʃən//

A conclusion reached on the basis of evidence and reasoning.

Our deduction is that the supplier sent the wrong parts.

Vocabulary

evidence

noun CEFR B2 //ˈɛvɪdəns//

Facts or information indicating whether a belief or proposition is true or valid.

The email is evidence that the meeting was rescheduled.