once 曾经，能用于过去完一成时吗？

"曾经可用于过去完成时，现在只可用于现在完成时。"

1、我记得曾经在图书馆看过你。

2、For example, I remember I once saw you in the library.

"once"用于句首时一般用过去完成时，中文意思是“曾经”，而"for once"意为“有一次”，而非“曾经”，所以正确的句子应该是 "Once for once," 或者 "Once again."

3、Once for once, I remember I went to the library and saw you there.

4、Once again, I remember I saw you in the library.