一、时间状语从句
由连接词1) when、while、before、after、since、until(till) 、as soon as、as, once, no sooner---than, hardly/scarcely-----when, 等引导 例如：When it rains , the children will play indoors .
No sooner had I got home than it began to rain.
The moment the bell rang, all the students stood at attention.
as ，when ，while区别: when可以和延续性动词连用也可以和短暂性动词连用；而while和as只能和延续性动词连用。He was speaking to a stranger while I saw him.(x)
1）若主句表示的是一个短暂性的动作，而从句表示的是一个持续性动作时，三者都可用。
He fell asleep when [while, as] he was reading. I met him when [as, while] I was taking a walk in the park.
若主、从句表示的两个正在变化、发展的情况时（同时进行的动作）含有“一边……一边”或as表示"随时间推移"之意思，通常要用as。He was singing as he was walking.她边走边唱。
2）若从句是一个短暂性动作，而主句是一个持续性动作，可以用as 或when 但不用while。如：When [As] he came in, I was listening to the radio. 他进来时，我在听收音机。
It was raining hard when [as] we arrived. 我们到达时正下着大雨。
3）若主、从句表示的是两个同时(或几乎同时)发生的短暂性动作，一般要用 as (也可用 when)。如：
I thought of it just when [as] you opened your mouth. 就在你要说的时候，我也想到了。
