before long與long before

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

2. long before
(1)老早，早就(單獨使用，表示比過去某時早得多的時間)。如：
She had left long before.她老早就離開了。
That had happened long before.那事老早就發生了。
注：long ago表示“很久以前”，指的是從現在算起的很久以前，通常與一般過去時連用：
I met him long ago.我很久以前就認識他了。
(2)在……的很久以前，在還沒有……的很久以前。如：
She had left long before his return [he returned].在他回來的很久以前她就走了。
He had worked in the factory long before he got married.在他還沒有結婚的很久以前他就在這家工廠工作了。
注：此時的主句謂語通常用過去完成時，但有時也可用一般過去時(因為 before已體現了動作的先后關系)。如：
This happened long before you were born.這事在你還沒出生以前很久就發生了。
It is [was, will be] long before......在……前需要很久。如：
It was long before he came back.過了好久他才回來。
It won't be long before we see each other.不久我們又會見面的。
比較：
It wasn't long before he realized his mistake. / Before long he realized his mistake.不久他意識到了自己的錯誤。