Learn English – Why do people say “I have known someone to do” but not “I know someone to do”

expressionsphrasessyntactic-analysis

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?

Best 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."

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