It was not long before they made their appearance.请详细解释一下这句话的结构及意思
问题描述：
It was not long before they made their appearance.请详细解释一下这句话的结构及意思
答
It was not long before they made their appearance.不久他们便出现了.
It主语
was not long 系表结构,谓语.
before they made their appearance.时间状语从句