knock on/at the door?

[Vidor]

You can't knock on a door from a distance.

[opa6x57]

I'm not a native speaker but in my opinion, "knock AT the door" implies the location of the person doing the knocking. In this case, it is somewhere close to the door or "at the door".

Therefore, I tend to think "he knocks at the door" as equivalent to "he stands in front of the door and he knocks"

As for "knock on the door", to me, it has a literal meaning of knocking the hand against the door. This phrase only describes the action of knocking. In common sense, it should therefore imply the location of the person albeit indirectly.

Exactly.

Unless he is extend-a-man with extremely long arms, to be able to knock on the door, he must be at the door, knocking.

I am an American English native speaker - 53 years old (not a teacher) and the reality is that these two phrases are functionally the same.

[5jj]

I am an American English native speaker - 53 years old (not a teacher) and the reality is that these two phrases are functionally the same.

I agree, and our feeling seems to be supported by the Corpus I mentioned in post #7.

It appears that some people prefer 'at' and others 'on'. Some possibly use both.

Unfortunately there also appear to be a number of users of only one form who believe that their preference is the only correct one. One of them even believes he knows what I would or would not say. I don't think that you, Vladimir and I are going to change any minds now.