Can’t and Couldn’t for Impossibility
Can’t - Present Impossibility
When we’re certain something is NOT true:
Negative Deductions
- That can’t be right. (I’m sure it’s wrong)
- She can’t be 60. She looks so young!
- He can’t be the thief. He was with me all day.
Expressing Disbelief
- You can’t be serious!
- This can’t be happening!
- It can’t be true!
Logical Impossibility
- He can’t be at work. It’s Sunday.
- They can’t be hungry. They just ate.
- She can’t be asleep. I can hear music.
Can’t Have - Past Impossibility
can’t have + past participle
When we’re certain something did NOT happen:
Negative Conclusions about the Past
- She can’t have failed. She studied so hard.
- They can’t have left already. It’s only 5 pm.
- He can’t have said that. He’s too polite.
Disbelief about Past Events
- You can’t have eaten everything!
- She can’t have forgotten. I reminded her twice.
- They can’t have known about the party.
Couldn’t - Another Option
“Couldn’t” can also express impossibility, slightly softer than “can’t”:
Present
- That couldn’t be the answer. (I doubt it very much)
- He couldn’t be guilty. (I find it hard to believe)
Past
- She couldn’t have done it alone.
- They couldn’t have known the truth.
Must vs. Can’t
| Must (positive deduction) | Can’t (negative deduction) |
|---|---|
| It must be true. | It can’t be true. |
| He must be tired. | He can’t be tired. |
| She must have known. | She can’t have known. |
Both express logical conclusions based on evidence.
Examples in Context
“He can’t be the winner. He didn’t even enter the competition! There must be a mistake. Someone couldn’t have registered his name without asking.”
Expressing impossibility and disbelief.
Can’t and couldn’t express near-certain impossibility or logical deduction in the negative: “She can’t be at home — I can see her car is gone.” This deductive use is the opposite of must (positive deduction).