have been? may have been?

mjeeem I want to know why 'have been' cannot be used in this part.

The main reason it can't be used is that the agreement between the subject and verb would be wrong.

There have been a real William Tell. Wrong agreement.
There has been a real William Tell. Correct agreement.

'William Tell' is singular. 'have'

Wow.. I didn't see it, thank you. But what if (A)is has been and (B) is may have been?
What is the difference between them?

mjeeemWow.. I didn't see it, thank you. But what if (A)is has been and (B) is may have been?What is the difference between them?

In that case, you would have this choice:

There has been a real William Tell, but he was also possibly a legendary hero.

This makes no sense because the first part contradicts the second part. The first part

Thanks, CJ. Your answer is very helpful!!
But I have another question.. I learned that the structure 'may/might/could have p.p' shows probability and possibility.
Then the sentences that use these structures doesn't mean real thing?

mjeeemthe structure 'may/might/could have p.p' shows probability and possibility.

Possibility. Correct. may have, might have, and often could have, all mean "It is possible that ... something happened (in the past)", that is, "Maybe ... something happened".

— I heard a noise in the back yard. I wonder wh

Thank you very much, CJ. Your explanation is very clear and helpful!