Can't have Vs couldn't have

I really can't understand the difference and the situation we use them. I put some example to make my problem clear.

Her favorite film is Kes, and she talks with awe about the moment when the boy finds the dead bird. She thinks, from the look on his face, that the child actor can't have known in advance that the bird was dead.

If he knew there were blacks in all the other countries, but he didn't know there were blacks in Brazil, he'd have to have thought Brazil was somehow an exception to the rule. But he can't have believed that. No sane person could. Brazil has the second-largest black population of any country in the world.

Please do let me know if you find problems [in the program] as I'm quite sure I can't have found every bug.

Tom can't have written this because it is in French and he doesn't know French.

[He] couldn't have known that just six hours later, he, the only male flight attendant on American Airlines Flight 11, would die suddenly when...
As a young woman who had never given birth before, she couldn't have known what the experience of childbirth would be like...

Athletic Director Edgar Johnson couldn't have known what he was getting into when he made that fateful decision to cut the Delaware wrestling program on June 18

I wish I had written Nowhere Man, by Aleksandar Hemon, but I couldn't have written it, because no one can write like Hemon.