【名师点睛】
用来修饰主句中的动词，副词和形容词的从句叫状语从句。根据其含义状语从句可分为时间状语从句，地点状语从句，条件状语从句， 原因状语从句，结果状语从句，比较状语从句，目的状语从句，让步状语从句。
1. 时间状语从句
(1)时间状语从句常用when, as, while, before, after, since, till, until, as soon as等连词来引导。例如：
It was raining hard when got to school yesterday.
While he was doing his homework, the telephone rang.
As he walked along the lake, he sang happily.
