Must and Can't
Using must and can't for logical deductions about what is certainly or impossibly true
Must = I’m Sure It’s True
When you look at a situation and there is only one reasonable explanation, use must.
You are not giving an order. You are saying: the evidence points in one direction, so this must be the case.
- You’ve been in meetings since 7 a.m. You must be exhausted.
- The sales team hits their targets every quarter. The manager must be doing something right.
- She does the same data entry task eight hours a day. She must get bored.
Can’t = I’m Sure It’s Not True
When something seems impossible based on what you know, use can’t.
- You just came back from lunch. You can’t be hungry already.
- They only moved to this office last week. They can’t know many people in the building yet.
The Structure for Present Situations
Must and can’t are followed by the base form of the verb. With adjectives or nouns, use be.
| Pattern | Example |
|---|---|
| must/can’t + be + adjective | He must be tired. / She can’t be serious. |
| must/can’t + be + -ing | You must be joking. / He can’t be working — his light is off. |
| must/can’t + base verb | They must know the answer. / She can’t have the key. |
Must Have / Can’t Have = Past Deductions
When you are making a deduction about something that already happened, use must have + past participle or can’t have + past participle.
- I can’t find the contract anywhere. I must have left it at the client’s office.
- The old headquarters was right next to the highway. It must have been noisy.
- She never replied to my email. She can’t have received it.
- He walked straight into a glass door. He can’t have been looking where he was going.
The Structure for Past Situations
| Pattern | Example |
|---|---|
| must/can’t + have been + adjective | It must have been stressful. / It can’t have been easy. |
| must/can’t + have been + -ing | She must have been waiting for hours. / He can’t have been listening. |
| must/can’t + have + past participle | I must have dropped it. / She can’t have got my message. |
Couldn’t Have = Same as Can’t Have
You can use couldn’t have instead of can’t have with no change in meaning.
- She can’t have got my message. = She couldn’t have got my message.
- He can’t have been looking where he was going. = He couldn’t have been looking where he was going.
Both forms say the same thing: based on the evidence, this was not possible.
Quick Self-Check
A colleague has been travelling all day and just arrived at the office. You say: 'You _____ tired.'
Select your answer:
That restaurant is always empty. It _____ very good.
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I can't find one of my files. I _____ it somewhere on the shared drive.
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The client never replied to our proposal. She _____ it.
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It rained every day during their business trip. It _____ very pleasant.
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