相信对你有帮助`` when, while ,as soon as 这三个词都有"当……时候"之意，但用法有所不同，使用时要特别注意。 ①when意为"在……时刻或时期"，它可兼指"时间点"与"时间段"，所引导的从句的动词既可以是终止性动词，也可是持续性动词。如： When I got home, he was having supper.我到家时，他正在吃饭。 When I was young, I liked dancing.我年轻时喜欢跳舞。 ②while只指"时间段"，不指"时间点"，从句的动词只限于持续性动词。如： While I slept, a thief broke in.在我睡觉时,盗贼闯了进来。 辨析 ①when从句与主句动作先后发生时，不能与while互换。如： When he has finished his work, he takes a short rest.每当他做完工作后，总要稍稍休息一下。(when = after) When I got to the cinema, the film had already begun.当我到电影院时，电影已经开始了。(when=before) ②when从句动词为终止性动词时，不能由while替换。如： When he came yesterday, we were playing basketball.昨天他来时，我们正在打篮球。 ③当从句的谓语是表动作的延续性动词时，when, while才有可能互相替代。如： While / When we were still laughing, the teacher came in.正当我们仍在大声嬉笑时，老师进来了。 ④当从句的谓语动词是终止性动词，而且主句的谓语动词也是终止性动词时,when可和as通用，而且用as比用when在时间上更为紧凑，有"正当这时"的含义。如： He came just as (or when) I reached the door.我刚到门那儿，他就来了。 ⑤从句的谓语动词如表示状态时，通常用while。如： We must strike while the iron is hot.我们应该趁热打铁。 ⑥while和when都可以用作并列连词。 Mike is tall while his brother is short.迈克的个子很高，然而他弟弟很矮。 Mary was on her way home when two big boys stopped her.玛丽正走在回家的路上，这时两个大个子男孩拦住她。 as soon as的用法： （1）、as soon as one can = as soon as possible 表示“尽可能快”之意，如： ① You must ring me up as soon as you can. 你必须尽快给我打电话。 ② Please write to me as soon as possible. 请尽早给我写信。 （2）、as soon as 引导时间状语从句，表示“一……就……”。如果主句用将来时或情态动词或祈使句，从句必须用一般现在时代替将来时。如： I’ll tell him the news as soon as he comes. 他一来我就告诉他那消息。 She may come as soon as she finishes the work. 事情一做完她就可以来。我找好久了
since是用于完成时,作“自从……”意思是有两种用法，1.例如：I have been a teacher since two years ago./ I was twenty./ I came here.（若要用动词，一定要用短动词的过去式)2.例如：It is/has been 2 years since I came here.(前面要用一段时间，后面用短动词的过去式)
when在用作“当……的时候”时应注意主句用将来时，从句用一般时。
If可引导条件状语从句，意为“如果”，它也可以引导定语从句的问句，此时，if和weather都可用，但当后面有or not时，就只能用weather,因为weather...or not是固定搭配！
例如：1.Can you tell me if she is here.
= Can you tell me weather she is here.
2.Can you tell me weather she is here or not.
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