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Hi,

What is the difference between saying someone 'could not know' something and 'could not have known' something?

Thank you.
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SuperESLSo when writing from the perspective of the past, "could not know" and "could not have known" can be used interchangeably? Thank you.
They are extremely similar, but I would hesitate to call them interchangeable in every one of the billions of sentences where they can be used. Nevertheless, it seems to me that they are virtually always interchangeable. I'd say that if you're in doubt, use 'could not have known'.

could not know is like 'had no way of knowing'.
could not have known is like 'it was logically impossible (for the subject) to know'.

Those are not really very different, are they? Here are some examples I found on fraze.it. (I probably would have used could not have known in all of these examples.)

He could not know it at the time, but his career was over.
What he did not know, could not know, was what lurked just a few yards away.
What Williams could not know was how his schedule would lighten the next week.
He could not know that, months later, flashing back to a sniper incident in Iraq, he would find home to be a deeply threatening place.
Eventually, though Hazlitt could not know this, she had a child by Tomkins and moved in with him.
A dialog box told me that new hardware had been plugged in, as though it was possible that I could not know that.
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Gull died before Albert Victor, and so could not have known about Albert Victor's death.
Reports at the time make clear the killers could not have known whether the figure was a man, woman or child.
He could not have known how basketball would change his life.
Investing in an NBA franchise became hugely profitable for him, but he could not have known that at the time.

CJ
Comments  
SuperESLcould not know'
Now or any time.
SuperESLcould not have known
Before; in the past.
Students: We have free audio pronunciation exercises.
So when writing from the perspective of the past, "could not know" and "could not have known" can be used interchangeably? Thank you.
 CalifJim's reply was promoted to an answer.