Hello, English learners! Welcome to a new grammar lesson. In this lesson, you will learn how to use must and can’t for logical conclusions about present and past situations. We will examine the correct structures, natural examples, common mistakes and ways to use this grammar in B2 First and C1 Advanced tasks.
What Is a Logical Conclusion?
A logical conclusion—or deduction—is an opinion based on evidence rather than direct knowledge.
Consider this situation:
The lights are off, the curtains are closed and nobody is answering the door.
From this evidence, you might say:
- They must be asleep.
You do not know for certain that they are asleep, but it seems like the most logical explanation.
Now imagine that you have just spoken to them on the phone and they told you they were at the airport:
- They can’t be at home.
Again, you are using evidence to reject a possibility.
Compare the two basic meanings:
Must for Present Logical Conclusions
We use must when evidence makes us almost certain that something is true.
| must + base verb | actions or states | You must know Daniel. |
| must be + noun/adjective/place | identity, condition or location | She must be the manager. He must be tired. |
| must be + verb-ing | an action happening now | She must be working. |
Compare:
- She must work in finance. → her usual job
- She must be working. → what she is doing now
Can’t for Present Logical Conclusions
We use can’t when evidence makes us almost certain that something is not true.
| can’t + base verb | actions or states | He can’t know the answer. |
| can’t be + noun/adjective/place | identity, condition or location | He can’t be the teacher. She can’t be hungry. |
| can’t be + verb-ing | an action that is not happening now | She can’t be sleeping. |
Must Have for Past Logical Conclusions
We use must have when evidence makes us almost certain that something happened in the past.
| must have + past participle | a completed past action | She must have forgotten. |
| must have been + noun/adjective/place | a past identity, condition or location | She must have been tired. |
| must have been + verb-ing | an action in progress in the past | He must have been working. |
Compare:
- He must have finished the report. → probably completed
- He must have been working on the report. → activity in progress; completion is unknown
Remember: Use must have, not must had.
Can’t Have for Past Logical Conclusions
We use can’t have when evidence makes us almost certain that something did not happen in the past.
| can’t have + past participle | a past action that probably did not happen | She can’t have written this message. |
| can’t have been + noun/adjective/place | a past identity, condition or location | He can’t have been asleep. |
| can’t have been + verb-ing | an activity that was not in progress | She can’t have been driving. |
Remember: Use can’t have, not can’t had.
Must, May, Might or Can’t?
These modal verbs express different degrees of certainty.
Consider this situation:
Julia is not answering her phone.
You could say:
- She must be in a meeting.
I am almost certain. - She may be in a meeting.
This is possible. - She might be in a meeting.
This is possible, but I am not certain. - She can’t be in a meeting.
I am almost certain that this is not true.
A useful scale is:
| Degree of certainty | Modal verb |
| Almost certain something is true | must |
| Something is possible | may, might, could |
| Almost certain something is not true | can’t |
Must and Can’t in Cambridge Exams
Knowing how to use must and can’t for logical conclusions can help you in several parts of B2 First and C1 Advanced.
Speaking
When comparing photographs or discussing possible situations, you can say:
- They must be colleagues because they are wearing the same uniform.
- The photograph must have been taken in winter because everyone is wearing heavy coats.
- They can’t be at home because the room looks like a laboratory.
- He must be explaining something to the other people.
These forms help you speculate without presenting every idea as a fact.
Writing
In essays, reports and reviews, deductions can help you explain evidence:
- The decline in attendance must be connected with the change in timetable.
- The new system can’t have been tested properly, given the number of technical problems.
- The author must have carried out extensive research before writing the novel.
Use these structures carefully in formal writing. They express strong conclusions, so the supporting evidence should also be strong.
Reading and Use of English
You may need to recognise or transform structures such as:
I am almost certain that Leo forgot the meeting.
- Leo must have forgotten the meeting.
Or:
It is impossible that Sara wrote this email.
- Sara can’t have written this email.
Using must and can’t for logical conclusions allows you to do more than make simple guesses. These structures help you examine evidence, reject unlikely explanations and communicate how confident you are about your conclusions.
Download Must and Can’t for Logical Conclusions – Practice Worksheet
Related posts:
- Modal Verbs of Deduction
- Modal Verbs May and Might
- Modal Verbs of Speculation about the Past
- How to Talk About Photos in B2 First Speaking Part 2
- Beyond Probably: Ways to Express Probability in English
- B2 First Speaking
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