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　　1. 易混引导词while, when, as的区别：

　　　　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。如：

　　　　When he came in, his mother was cooking.

　　　　When (While) we were at school, we went to the library every day.

　　　　when从句的谓语动词可以在主句谓语动作之前、之后或同时发生;while和as从句的谓语动作必须是和主句谓语动作同时发生。

　　　　While表示时间段,因此，while 从句的谓语动词要用延续性动词。如：

　　　　Please don't talk so loud while others are working.

　　　　As与when用法相似，但着重强调主句动作与从句动作同时发生，有"随着……"或"一边……一边……"之意。如：

　　　　As you get older, you get more knowledge.随着年龄的增长，你获得的知识就越多。

　　　　在将来时从句中，常用when，且从句须用一般时代替将来时。

　　　　①You shall borrow the book when I have finished reading it.在我读完这本书后，你可以借阅。

　　　　②When the manager comes here for a visit next week，Ill talk with him about this.下周，经理来这参观时，我会和他谈谈此事。

　　　　when用于表示“一……就……”的句型中(指过去的事情)。

　　　　sb.had hardly(=scarcely) done sth.when...=Hardly / Scarcely had sb.done sth.when...

　　　　①I had hardly /scarcely closed my eyes when someone knocked at the door.=Hardly / Scarcely had I closed my eyes when someone knocked at the door.我刚一闭上眼，就有人在敲门了。

　　　　②I had hardly /scarcely entered my room when the telephone rang.=Hardly /Scarcely had I entered my room when the telephone rang.我刚一走进房门，电话就响了