(A)before long表示“很快;不久”之意，其中before是介词，而long是名词。before long可与过去或将来时态的谓语动词连用。例如：

1)My book will be published before long.我的书不久就要出版。

2)I received his letter before long.我很快就收到了他的信。

3)We’ll see that film before long.我们不久就要看那部电影。

4)Before long they found the lost paper.不久，他们就找到了丢失的文件。

5)Before long she came running behind me.很快，她就跑着追我来了。

(B)long before是“很久以前”之意。其中before为副词，long也是副词。一般用在谓语动词为过去完成时的句子里，有时也可用在谓语动词为一般过去时的句子中。例如：

1)He said he had seen the film long before.他说他很久以前就看过这部电影。

2)I told him I had been to Beijing long before.我告诉他，我很久以前就到过北京。

3)I had known her long before.很久以前我就认识她了。

4)She died long before the war.早在战前很久她就去世了。

long before后面也可以跟从句。这时，before为连词。例如：

1)I had heard about you long before I met you.与你见面之前我就久闻你的大名了。

2)I had read the novel long before you read it.你看的这部小说我很久以前就看过了。

3)I knew Mr Smith long before I knew you.早在认识你之前我就认识史密斯先生了。