ago , before的用法和区别
ago 立足于现在，表示从现在起，若干时间之前；ago 通常与一般过去时连用不能与完成时连用；
I met him three years ago .
（距今）三年前，我遇到他。
I met her once several years ago.
几年前我曾见过她一面。
They checked out ten minutes ago.
他们十分钟前就结账走了。
He and his wife busted up three months ago.
他和他妻子三个月前就关系破裂了。
before 立足过去，表示从过去某一时刻起，若干时间以前。before通常与过去完成时连用。
I had met him three years before .
（距当时）三年前我见到他一次。
Write her before you go.
你去之前先给他写封信。
I have seen him before.
我以前曾经见到过他。
You should hibernate before I contact with you.
在我与你联系之前，你得潜伏不动。