before long相当于soon, a little later,意为“不久或不久以后”，常与将来时、过去时态连用，在句中作状语。如：

The book will be published before long. 那本书不久将要出版。

Before long he came to Canada to go on with his education. 不久以后他去了加拿大继续深造。

long before相当于long, long ago意为“很久；很久以前”，常用于过去时或过去完成时。如：

Long before, there is a big museum here. 很久以前，这里有一座大博物馆。

I had heard of him long before I came here. 我来这儿很久以前就听说过他的情况。