【正文】 when, while, as表示主句谓语作和从句的谓语动作同时发生或几乎同时发生。当他们还在说笑的时候，老师进来了。他正要上床，忽然门铃响了。尽管可以打的，但他还是步行。我喜欢英语而他却喜欢数学。as和when两者经常可以通用。As time goes on, it39。We listened to the singer sing as he played the guitar.【注意】as 引导时间状语从句表示“当……的时候”“在……期间”时，该从句的谓语通常是那些表示动作意义的动词，而不能是be(是、在), have(有)，like(喜欢)，belong(属于)等表示状态或静态意义的动词。（从句动作发生在主句之后，注意时态）c．while从句只能表示延续的动作或状态或主从句中动作的对比。无论你何时来都欢迎。I go home.(=not……until……)提示：I’ll water the flowers before it gets dark. 我要趁着天还没黑把花浇好水。④It wasn’t long before+ 一般过去时：没等多久就……It was not long before I realized I was wrong.放学后我们踢足球吧。2． Not……until /till……表示直到……才……People do not know the value of their health till until before they lose 。②not until 句型的强调句：在强调句型(It is/ was not until…….that……)中一般只能用 until， 不用till。It is three years since he began to 。3．since还可以用作副词或介词。On arriving home he called up Lester.=As soon as he arrived home, he called up F． immediately instantlyTell him I need to see him the minute he ，他一到我就要见他。位于句子的开头时，主句须用倒装语序。一旦开了头，你就应当继续下去。The last time I spoke to Bob, he seemed happy ，他看上去很开心。(从句为一般现在时，主句要用将来完成时)。Any time you e to London do look me up. 你无论什么时候到伦敦来，一定要来看我。你一旦见过他，就不会忘了他的。她刚要睡着，忽然敲门声把她吵醒。Telephone me the moment (that) you get the results. 你一有结果，马上给我打电话。They phoned her immediately they reached 。My uncle went to Tibet in the 1950s. He has been living there（ever），从那以后他一直生活在那里。Tom is now working on the farm. It’s two years since he was a college ，他大学毕业已有两年了。It was not until she took off her dark glasses that I recognized ，我才认出她。I could hardly understand the theory until my teacher explained it to 。Not 有时被no, nobody, nothing, never, hardly, little 等否定词所取代。暴风雨过后不久，太阳出来了。其他变形句型：The war last three years before it finally 。【必背】before 常用句型 ①It will be + 时间段+before+ 一般现在时：要等多久才…….作此意时，从句中常见情态动词can/ could。每当我
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