ever ----曾经 多与完成时连用
I have ever been to Beijing .我曾去过北京.
once-----曾经（有一次）,多与过去时连用
We lived in London once .我们曾在伦敦住过.
It once belonged to my father.
这个曾经属于我父亲.
I once went around the world.
过去我周游过全球.