状语从句在复合句中作状语，修饰主句中的谓语动词、副词、形容词或整个句子。根据句子的含义，它可表时间、地点、原因、目的、结果、条件、行为方式、比较、让步等。状语从句通常由一个连词引起，有时由一个起连词作用的词组引起。
时间状语从句
I) when，while和as
He entered the room when (while， as) the meeting was going on.
when 通常指一时的动作或状态，可表示从句动作与主句动作同时发生，或先于主句动作；while常用来引导两个同时进行的持续时间较长的动作；as常用来引导两个动作紧接着发生，强调“同一时间”或“一前一后”。
When I meet the right chance， I will try my best.
While the teacher explained the text， the students listened attentively and took notes
He sang songs as he was taking a bath.
As spring warms the earth， all flowers begin to bloom.
注：when有时作并列连词意为“正在这时”、“突然”，在句末，不能位于句首。从句的谓语动词常用一般过去时，主句通常用过去进行时、过去完成时或be about to结构等。
She was about to leave when Mark arrived so she stayed a little longer.
He had been walking under the trees when an apple fell down.
Ⅱ) as soon as， hardly (scarcely， barely) … when… ， no sooner …than… 等引导的时间状语从句，意为“ 一… 就 …”
Jack had scarcely /hardly seen me when he left the room.
The spy had no sooner returned home than he was told to go to another country.
那个间谍一踏进国门就奉命要去他国。
注： 在no sooner … than， hardly … when …引导的时间状语从句中，若把no sooner， hardly， scarcely， barely 放在句首，则要用倒装结构。
No sooner had he heard the news than he fainted.
Scarcely had he entered the room when the telephone rang.