由when,before, after,until和as soon as等连词引导的从句称为时间状语从句。

①When we left,it began to rain.当我们离开的时候，天开始下雨了。

②You must look around before you cross the road.穿过马路前，必须观望一下再过。

③Please give me a ring after class.下课后，请给我打个电话。

④We will wait here until the teacher comes back. 我们将一直在这等直到老师回来。