想翻譯呢句野做英文"有人曾經告訴我,但已經是過去的事情"

"有人曾經告訴我,但已經是過去的事情"
Someone had told me but it was in the past.

But I think it's better this way in English:
I've been told about that once, but it was history now.

Though somebody has told me about the matter but that was already over.

其實用once不會怪，英語人常會這樣講「曾經」的，只不過我們中文人學的英文時只懂它(once)的一個意思=一次，所以才覺怪。不過最大的問題是後半句的「過去的事情」寫成in the past或類似的用詞。我相信你的意思是等於「過去有人曾經告訴我...」但是其實沒有講那內容，那告訴你內容應該不是那「過去的事情」吧？
我只是想指出你寫原來的中文句本身已經有點難度(問題)，不過就算按你的修辭來譯也可以是

Someone had told me once, which was some time ago.

我相信這是較為接近英語人的寫法，像寫小說的手法，製造層次感。
並且這句英文亦同樣反照你原文其實沒有講出究竟告訴了你的是甚麼，同樣給人一種說了出來但卻又空洞感覺，只表達這「告知」是在以前發生的。

希望這是你期望的答案。

Someone had told (noticed) me before in the past (previously).

有人曾經告訴我,但已經是過去的事情=有人曾經在過去告訴我
Someone once told me that in the past.
有人曾經告訴我,但已經是過去的事情=有人曾經告訴我一件在過去的事情
Someone once told me that, but that was in the past.