常用引导词：when, as, while, as soon as, while, before, after, since , till, until
特殊引导词：the minute, the moment, the second, the instant, every time, each time, any time , the first time, the last time, the next time , immediately , directly, no sooner … than, hardly …when, scarcely … when
1 【区别】when, while和as的区别
（1）when引导的从句的谓语动词可以是延续性的动词，又可以是瞬时动词。并且when有时表示“就在那
时”。 When I lived in the countryside, I used to carry some water for him.当的住在农村时，我常常为他担水。（延续性的动词）
