What is the difference between can't have done and could have done in British English?
I want to know what is the difference between “can't have done” and “couldn’t have done?”

Sentence 1. She can't have gone to bed. It's only ten o'clock.
Sentence 2. She couldn't have gone to bed. It's only ten o'clock.

There’s no doubt that these sentences are very similar and they are both correct. Some people think that the first sentence is slightly stronger in that you are saying that (in your opinion) it's almost impossible that she has gone to bed and the second sentence leaves open the small possibility that you are wrong about this!

Personally, I don’t think that’s the case.

What do you think?

Question

What is the difference between can't have done and could have done in British English?
I want to know what is the difference between “can't have done” and “couldn’t have done?”

Sentence 1. She can't have gone to bed. It's only ten o'clock.
Sentence 2. She couldn't have gone to bed. It's only ten o'clock.

There’s no doubt that these sentences are very similar and they are both correct. Some people think that the first sentence is slightly stronger in that you are saying that (in your opinion) it's almost impossible that she has gone to bed and the second sentence leaves open the small possibility that you are wrong about this!

Personally, I don’t think that’s the case.

What do you think?

Answer

I would use the sentences in slightly different situations e.g.

a) It is now 10 o'clock.  Where is she now? I don't know but she can't have gone to bed. It's only ten o'clock.

b) It is now 11 o'clock.  She is in bed.  We don't know precisely when she went to bed and we are speculating about the time. She couldn't have gone to bed at ten o'clock because she always eats dinner at 10.15.

In b), I think I would accept "can't" as well but not both options in a).

There might be something in "Advanced Grammar in Use" by Hewings on this, but I haven't checked.

Answer

This is at least the third time you've asked a variation of the question.

And every single time, people have told you that they mean approximately the same thing and that the difference between the two is tiny.

Tomorrow, they will still mean approximately the same thing.  They will still mean approximately the same thing if you ask another 4 times.  And the same will be true next week and after another 10 repeats of the question.

Is there a reason you're obsessed with these two sentences?