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when while as after before as soon as since till /until by the time 在时间状语从句中，要注意时态一致。一般情况下主句是将来时的时候，从句要用一般现在时。

when,while和as的区别

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。

when有时表示“就在那时”，when=and then； at that moment （正在那个时候）； 还可以表示从句动词的动作在主句动词的动作“之前 ”或“之后”发生。有很多情况已经约定俗成，可以当成句型来记忆。例如：

be going to do …… when ……did ……

be about to do…… when…… didi……

be on the point of doing…… when ……did……

be starting to do…… when …… did……

be doing…… when …… did……

had just done……when…… did……

had hardly done……when……did……

had scarcely done……when……did……

had no sooner done…… when…… did……

When she came in,I stopped eating. （瞬时动词）

When I lived in the countryside,I used to carry some water for him.（延续性的动词）

We were about to leave when he came in.

When I had read the article,he called me. （ 从句动作发生在主句之前，注意时态表达，只能用when ）

When I got to the cinema,the film had begun. （从句的动作发生在主句之后，只能用when，并要注意时态）

He was about to leave,when the telephone rang. （ 此时不能放在句首。主句动词一般表达 “正在” “即将”。at the moment/and then）