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when引导时间状语从句
　　when是引导时间状语从句最常用的一个词,它涵盖的时间范围比较广泛,在作“当……的时候”解时,有下面几种常见用法：
　　1。表示在某个具体的时间里发生的动作。此时,when不能用as或while替换。
　　When she comes,I shall tell her to wait for you。她来的时候我会告诉她等你的。(指具体的时间点)
　　When his father died of illness in 1980,he was only two years old。1980年他父亲病逝时他只有两岁。(指1980年这个具体的时间点)
　　2。表示一个动作与另一个动作在同一时间发生,或一个动作在另一个动作所延续的时间范围之内发生。此时,when可以用as或while替换。
　　I hurt my shoulder when (while,as) I was playing basketball。我打篮球时弄伤了肩膀。
　　He entered the room when (while,as) the meeting was going on。正当开会的时候他走进了房间。
　　When (While,As) Tom was reading,Jim was writing。汤姆看书的时候,吉姆在写东西。
　　两个同时发生的动作,如果不是一次性而是重复性的,即过去或现在的习惯动作,可用连词when或whenever,意思是“每当……的时候”。例如：
　　When (Whenever) he goes to town,he always visits his aunt。每当他去镇上,他都去拜访他姑姑。
　　She always felt ill when (whenever) she ate oyster。她每次吃牡蛎都会生病。
　　在上述两例中,不管是用when还是用whenever,主句动词和从句动词或者都用一般现在时,或者都用一般过去时,分别表示现在或过去的习惯动作。
　　3。表示从句的动作发生后,主句的动作立即发生。此时,when表示动作的先后顺序,相当于after,不能用as或while替换。
　　When he had done his homework,he watched television。=After he had done his homework,he watched television。他做完作业后看电视。
　　When he had painted the windows,he painted the doors。=After he had painted the windows,he painted the doors。他把窗户油漆完后,又把门油漆了。
　　在这里,when从句的谓语动词都用了完成体,when从句既可以置于句首,也可以置于句尾。但是,如果when从句的谓语动词用一般现在时或一般过去时,when从句只能位于句首,表示从句动作先于主句动作。
　　When he had done his homework,he watched television。=He watched television when he had done his homework。
　　When he had painted the windows,he painted the doors。=He painted the doors when he had painted the windows。
　　但是,“When he painted the windows,he painted the doors。”这个句子则不能写成“He painted the doors when he painted the windows。”
打字不易,