passive voice

It is known where it came from .

2007-04-02 17:53:41 補充：
更正 :It is not known where it came from .it 是 where it came from 的先行主語 .where it came from 是名詞子句 .is not known 是動詞被動 form .no one 於被動語態可以省去 .

It is unknown to every one that where it came from.

no one knows where it came from

object是"no one",
当变成passive form时，
no one就要变成subject :

It comes from a place which is known by no one.

在这里，绝对不能用past tense，
因这是事实，用回simple present tense。

Its origin is unknown.

It was come from somewhere no one knows.

It's origination* is unknwon

it is come from an unknown place

2007-04-02 17:27:05 補充：
sorry, I overlook the "tense"It should be It WAS come from an unknown place.