[注意]下列情况只能用whether不能用if：①引导主语从句,②引导表语从句,③引导从句作介词宾语,④引导不定式短语,⑤引导让步状语从句,⑥在动词discuss之后,⑦在wonder / not sure之后,⑧在if与whether含义易混时。如：Whether it is a fine day next Sunday is still a question.(下个星期天是不是个好天还是个问题)(引导主语从句) / Please ask him whether to go there with a raincoat or not.(请问一下带不带雨衣去那儿)(作动词的宾语) / Hainan is the place to be, whether it’s summer or winter.(海南是个该去的地方无论冬夏)(引导让步状语从句)/ Please let me know whether you need my help.(请告诉我你是否需要我的帮助)(引导宾语从句)(如果换成if则还可能表示“如果你需要我的帮助请告知”)
(3)while、when、as的用法区别：while常表示一个较长的动作,它引导的从句动作与主句的动作是同时发生的、是平行的;when可以表示较短的动作也可以表示较长的动作,主句和从句的动作可以同时发生也可以先后发生;as与上两词同义,可替换while和when, 表示主句和从句的动作同时发生,常译为“一边……一边……”。如：Please do not trouble me while I am writing my homework.(我写作业时请不要打扰我)/ I’ll go home when I have finished my job.(我干完了活儿就回去)/ They were running quickly across the road when they heard the sound of a truck coming.(他们正快速地穿过马路忽然听到了卡车开来的声音)/ As we walked in the dark street, we sang songs and talked loudly.(当我们在黑洞洞的街上走路时我们高声地唱歌说话)
(4)till/until与not…till/until的区别：前者表示一个延续性的动作,后者表示一个才开始的动作。如：I will stay here and watch the baby until you return.(我会呆在这里看着娃娃直到你回来)(stay这个动作一直进行到你return) / They won’t go on working until they get what they think is reasonable.(他们要到获得了他们认为合理的东西时才会继续干下去的)
另外till与until基本可以互换,但是在句首时只能用until,不能用till. 如：Until the last minute of the match we kept playing.(我们坚持到比赛的最后一分钟)/ Not until he had finished his work did he go home.(直到做完工作他才回家)(倒装句)
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