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.
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As for the difference, the first 'once' says 'on one particular occasion' to me, whereas the second 'once' is the 'once' of 'Once upon a time', namely, 'There was an occasion when ...'.
CJ
Comments
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.
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 almost tantamount to
I saw him.
Is this intuition correct?
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)?
Nevertheless, "at a party" seems like a perfectly normal way to end the sentence. It is acceptable, of course, but not the only structure that is acceptable.
CJ
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]