befor long shortly after如何区别?

before long：一个固定短语,常用作时间状语.常用于将来时态.
如：
He will come back before long.
Shortly after=soon after: after后常接名词或从句.
如：
Shortly after he came, we began our work.
By then, some plants will be flowering shortly after Christmas.
到那时,某些植物将在圣诞节后不久就开花.