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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?

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    Good question. I believe that this is merely customary, but you almost certainly should get a better answer than that. Welcome to EL&U. – Robusto Dec 23 '18 at 15:25
  • Comments are not for extended discussion; this conversation has been moved to chat. – Andrew Leach Jan 5 at 18:01
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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."

  • While I agree with the distinction you make with tenses, that is not the crux of my question. The fact that I know him to get up early returns exactly naught troubles me. – Eddie Kal Dec 23 '18 at 17:50
  • That's way too narrow. Here. Contrast sentences like "I know him to be rich/evil/sick" with "I've known him to be rich/evil/sick." – Tushar Raj Dec 23 '18 at 17:54
  • "Know someone/something to be" is a different story. As evidenced by the title, there is no question there to begin with. – Eddie Kal Dec 23 '18 at 18:18
  • "What is special about the structure I have known someone to do something that validates it as grammatical?" Nothing. Both constructs are grammatical and have distinct meanings, as I have demonstrated. Evidently one of them is significantly less popular, but usage patterns have nothing to do with grammatical validity. If you don't like be rich, you can try similar experiments with things like get angry when... or have patience and so on. Your OP was about grammar and meaning, so I answered keeping that in mind. I can't vouch for usage. – Tushar Raj Dec 23 '18 at 18:31
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    @FumbleFingers This is a very helpful comment. – Eddie Kal Dec 23 '18 at 21:37
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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.

  • This doesn’t really explain anything – it’s just a description of how the phrase is used, not a reason why. There’s no reason you shouldn’t be able to use the same construction to express your experience in the present tense. “I’ve known John to go fishing on Sundays” means roughly “I’ve known for a while that John often goes fishing on Sundays”; so why not “I know that John often goes fishing on Sundays” in the present tense? That is the question which was asked. – Janus Bahs Jacquet Dec 24 '18 at 9:07

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