I once saw vs I have seen once.

Hi iasadih;

Please give entire sentences. It's hard to tell with phrases.

These have the same meaning/ Once = (at) one time.
I have seen him once.
I once saw him.

I have seen him once.
I once saw him. I would say 'I saw him once.'

The sentences as you wrote them are already complete. On second thought, yes, "once" means "one time" in either case.

Still, something appears different.

I have seen him once.
I once saw him.

Firstly, sentence stress is different.

Secondly, in the Past sentence, "once" seems to be just a decoration, i.e.

I once saw him.

is a

iasadihI have seen him once.

To my ear, this strays slightly from idiomatic speech.

I have seen him, but only once.
I have only seen him once.

iasadihI once saw him.

This sounds incomplete to my ear.

I once saw him give money to a beggar.
I once saw him laugh behind her back.
_______________

That's what I meant: "once" has different meanings.

Concerning "I once saw him",

would it sound okay if supplemented with "at a party" (which is just a cosmetic change) or do you mean that it is "see sb do sth" that is acceptable here (I wonder why that could be)?

iasadihConcerning "I once saw him", would it sound okay if supplemented with "at a party" (which is just a cosmetic change)

Given that it expresses the specific circumstances under which you saw him, I don't know what you mean by 'cosmetic change'.

Nevertheless, "at a party" seems like a perfectly normal way to end the sentence.

iasad

Let me make my point more clearly, then.

If a sentence

I once saw him at a party

is acceptable, then without at a party, it should be considered okay as well.

[I am referring to your comment that I once saw him sounded incomplete]

If a sentence
I once saw him at a party
is acceptable, then without at a party, it should be considered okay as well. Not necessarily, that's not common or natural English.

Okay, so how should

I once saw him

be corrected to sound natural, with minimum necessary modification?

iasadihOkay, so how shouldI once saw himbe corrected to sound natural, with minimum necessary modification?

I saw him once.