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when可指时间点，可指时间段，从句中用延续性动词，也用非延续性动词；表示主句动作发生在从句较长动作过程中，when, as均可使用。从句中为be动词，用when；when说明从句的动作和主句的动作可以是同时发生，也可以是先后发生。由when引导的时间状语从句，主句用过去进行时，从句应用一般过去时。
when的用法句型
1.be doing sth…when…
例：He was still smiling when the door opened and his wife came in.
他正笑着的时候门突然开了，他妻子走了进来。
2.be about to do sth…when…
例：We were about to start when it began to rain.
我们刚要出发，天就开始下雨了。
3.had not done sth…when…/hardly…when…
例：He had not fallen asleep when the telephone rang.
他刚要入睡电话就响了。
4.had just done…when…
例：I had just gone to bed after a very hard day when the phone rang. 在劳累了一天之后我刚刚就寝，电话铃就响了。