[初三] 辨析 before long，long before

before long是个介词短语，在句中作状语，意为“不久以后、很快”，多与将来时或过去时连用。如：

I shall see you again before long．我很快就会再见到你的。

Before long he had to move on．不久他又得搬迁。

long before是个副词短语，意为“很久以前”，多与完成时连用。如：

He had taken a doctor's deGREe long before．他很久以前就获得了博士学位。

before不仅可以用作副词，还可以用作介词或连词，这时long before的意思为“早在……之前”。如：

He had come to Europe long before the war．他早在战争之前就来到欧洲。

He had read the book long before I readit．他早在我读这本书以前就读过它了。

He was just falling asleep when there was a knock at the door．他刚要入睡，就有人敲门。