before long与long before

1.before long不久，很快。如：
I'll be back before long.不久我就回来。
Before long he got married.不久他就结婚了。

2. long before
(1)老早，早就(单独使用，表示比过去某时早得多的时间)。如：
She had left long before.她老早就离开了。
That had happened long before.那事老早就发生了。
注：long ago表示“很久以前”，指的是从现在算起的很久以前，通常与一般过去时连用：
I met him long ago.我很久以前就认识他了。
(2)在……的很久以前，在还没有……的很久以前。如：
She had left long before his return [he returned].在他回来的很久以前她就走了。
He had worked in the factory long before he got married.在他还没有结婚的很久以前他就在这家工厂工作了。
注：此时的主句谓语通常用过去完成时，但有时也可用一般过去时(因为 before已体现了动作的先后关系)。如：
This happened long before you were born.这事在你还没出生以前很久就发生了。
It is [was, will be] long before......在……前需要很久。如：
It was long before he came back.过了好久他才回来。
It won't be long before we see each other.不久我们又会见面的。
比较：
It wasn't long before he realized his mistake. / Before long he realized his mistake.不久他意识到了自己的错误。