Could've done vs. Can't have vs. Couldn't

Negative:
I couldn't have done it.
I can't have done it.
I couldn't do it.
Positive:
I could do it.
I could have done it.

You base on their contexts:

• I couldn't have done it. - This is not very popular, it emphasizes the disability more than I can't have done it can

Wow, you've explained it so perfectly! I am grateful as the others, as I expect

Thank you very much.

I can't have done it.

1. The only conclusion to be drawn is that I did not do it.
The innkeeper can't have killed the old widow because he was seen on vacation in France at the time of the murder.
___

I couldn't have done it.

1. It would not have been (logically) possible for me to do it.
That dog couldn't have caught the bird in the t

CJ, you just made me a broad smile of joy on my face when I saw your breath-taking, comprehensive and apt explanation!

Thank you very much, it must have been really hard work to list nearly all possibilities of these modals.

And note what I've just learned: "I couldn't have written that as perfectly as you did!"

MaroldI couldn't have written that

Perfect English! It looks like my effort is paying off.

"I couldn't have written that."

But specifically having a look at this sentence, I have still little doubts that I wouldn't be able to explain why "couldn't" cannot be used or rather with which difference comparing to "couldn't have"? As it has been all written above, both these modals can take the meaning concerning the ability, talent...So what would be the difference exac

It is my understanding that in BrE can't have can also be equivalent to must not have. For example,
- She walked right past me and didn't even turn around; she can't have seen me (= she must not have seen me).
Is this correct?

H.

Maroldto explain why "couldn't" cannot be used or rather with which difference comparing to "couldn't have"?

For this situation,
"couldn't have written" will be taken to mean "would not have had the skill to write".
"couldn't write" will be taken to mean "failed to write what I tried to write".

The second is not appropriate in this conversatio

And it is me who is so grateful for this forum, since nothing can be compared to this - it is free for everyone and people are willing to help out.

Given the opportunity to attend this forum almost daily, I cannot even express myself how much it means to an english enthusiast as I am, to be surrounded by the best english teachers and experts which advices and counsels I frankly bear in mi

Henry74in BrE can't have can also be equivalent to must not have.

That's correct, except that Americans also use "can't have" with that meaning, and it is not another meaning. It is the same meaning that I have already listed above.

I must not have done it ~ I can't have done it ~ It is impossible that I did it.

Of the three forms ab