It couldn’t be true.

[thehammer]

Hello there, I found the following sentence from here. https://dictionary.cambridge.org/grammar/british-grammar/could

1- She must have made a mistake. It couldn’t be true.

can I use cannot instead?

Here are some more examples.

2- Sam: Oh Diane, have you heard? Marsha is pregnant with John's baby!"
Diane: No! It can't/couldn't be true!"

3- John: Peter died last year in a car accident. Me: What!! It couldn't/can't be true.

 

[5jj]

1. As we have no context, we cannot say which modal is appropriate.
2, 3. Use can't, not couldn't.

 

[jutfrank]

She must have made a mistake. It couldn’t be true.

I can't make sense of this.

It seems that the dictionary is suggesting that It couldn't be true is an alternative to It can't be true. If so, that's wrong.

 

[5jj]

She must have made a mistake. It couldn’t be true.
I can't make sense of this.

It's just about possible with couldn't as a distanced form of can't.

 

[jutfrank]

it's just about possible with couldn't as a distanced form of can't.

I don't follow.

Do you mean that for deductions/speculations about the present it's permissible to use couldn't instead of can't? Could you provide an example that I can make sense of?

 

[Barque]

She must have made a mistake. It couldn’t be true.

I took it as a description by a writer of someone's thoughts.

Jane's mind was in a whirl.
"Did he just refer to John's wife? John's married?" (Quoting her thoughts)
No, she must have made a mistake. She must have misunderstood. (Stating what she was thinking)
"It couldn't be true. It must be a different John." (Quoting her thoughts)

 

[jutfrank]

I took it as a description by a writer of someone's thoughts.

Jane's mind was in a whirl.
"Did he just refer to John's wife? John's married?" (Quoting her thoughts)
No, she must have made a mistake. She must have misunderstood. (Stating what she was thinking)
"It couldn't be true. It must be a different John." (Quoting her thoughts)

Interesting, but I don't really understand. Are you saying that 'It couldn't be true' is another way of saying 'It can't be true'?

Are you sure you're not using the past tense couldn't here because of the past narrative of the text? The writer is writing in the past tense, so the thought 'It can't be true' is accordingly backshifted to fit the narrative.

 

[Barque]

Are you sure you're not using the past tense couldn't here because of the past narrative of the text?

I suppose I am. I mean that I interpreted the OP's sentence the same way. I'm not saying "couldn't be true" and "can't be true" can always substitute each other.

 

[5jj]

I don't follow.

Do you mean that for deductions/speculations about the present it's permissible to use couldn't instead of can't? Could you provide an example that I can make sense of?

As you couldn't make sense of the first example thehammer gave us, I won't bother trying to come up with a better one.