Why do people say "I have known someone to do" but not "I know someone to do"?

Question

The structure I have known someone to do something is apparently considered grammatical and idiomatic. Examples from Google:

I have known people to take shops, put in a few articles and, without opening the doors, to sell the premises for as much as £1,000. (New Zealand Parliament)
I have known people to temporarily lose interest in sex and believe the problem was in their marriage but later realize that it was grief.
I have known people to lose battles with brain tumours, cancer, to become disabled, and to become very ill.

In contrast, I know/knew someone to do something is not a thing that people say. Apparently you can't say "I knew some people to drink coffee around midnight.*" Why is that? What is special about the structure I have known someone to do something that validates it as grammatical?

James Axsom · Answer

To answer why is a context about time.

If I say, “I have known someone to do”, then I’m reflecting on my experience, memory and past. The details of such a context tend to be abstract more often than not.

For example: “I have known John Doe to go fishing on Sundays.” This is reference of memory over a long period of time. John typically goes fishing on Sundays that I can remember over the past 5 years. The sentence merely shortens time as an abstraction of unknown amount of time.

On the other hand, “I know someone who may fish with you” is more in the present moment or immediate future.

For example: You are going fishing this afternoon or tomorrow? Well I know someone who may join you.

Tushar Raj · Answer

There is a difference in what you want to imply. Have known communicates that you are aware of at least some cases in which your statement has been true in the past. (It may or may not be true currently). Know means you think it's true in general.

Compare:

I've known him to get up early in the morning (= He's capable of it. He might do it again, even if he has possibly stopped.)

to

I know him to get up early in the morning (= He ususally does it.)

The second is in the same vein as "I know him to be a nice guy."

msh210 · Answer

When someone is saying this sentence, he doesn't mean "I now know that she regularly does this" (in which case know would fit) but "I have at various times known that he did it at those respective times", meaning (for example) "I knew last Tuesday that he did it at that time, and I knew three weeks ago that he did it three weeks ago". You're saying what you knew at the time (for each of those various times) rather than what you know now. So have known fits.
But that just pushes the question back: okay, so why do I say what I knew at various times instead of what I know now? I think that's because it's much more persuasive. To make a claim of what you know now about past actions — well, okay, you say so, but on what basis? But to make a claim of what you have known when it happened: that's a claim based on contemporaneous knowledge and thus much stronger.
That's my impression, anyway.

Why do people say "I have known someone to do" but not "I know someone to do"?

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