非限制性定语从句

性定语从句的五个“不能”
一、 非限制性定语从句通常不能用that引导。如：
1. I like the book, which was bought yesterday. 我喜欢这本书，这是昨天买的。
2. I like the book which / that was bought yesterday. 我喜欢昨天买的那本书。
二、 非限制性定语从句不能用why引导。要用for which代替why。如：
1. I had told them the reason, for which I didn't attend the meeting. 我已经把理由告诉了他们，为此我没有去开会。
2. I had told them the reason why I didn't attend the meeting. 我告诉了他们我不去开会的理由。(限制性定语从句“the reason why...”是常见搭配。)
三、 非限制性定语从句置于句首时，不能用which引导。关系代词as引导非限制性定语从句位置比较灵活，可以放主句前，也可以放主句后。如：
1. As I expected, he didn't believe me. 正如我所预料的，他不相信我。
2. She heard a terrible noise, which brought her heart into her mouth. 她听到一个可怕的声音，这让她把心都提到嗓子眼了。
四、 非限制性定语从句由“介词+关系代词”引导时，其中的关系代词不能用as。如：
1. He bought the car for more than $20,000, with which his father was angry. 他花两万多美元买了这辆车，他父亲对此很生气。
2. I finished my work ahead of time, after which I sat some time reading the newspaper. 我提前干完了我的活，随后我坐着看了一会儿报纸。
五、 在非限制性定语从句中，指人的关系代词作宾语时，只能用宾格whom; 不能用who替换，也不能省略。如：
1. Do you know Tom, whom we talked about? 你认识汤姆吗? 我们谈到过他。
2. This book, which you can get at any bookshop, will give you all the information you need. 这本书会给你提供所有你需要的资料，它在任何一家书店都能买到。(非限制性定语从句中引导词在从句中作宾语，无论指人还是指物都不能省略。)
as与which引导非限制性定语从句的区别
这类定语从句只能由 which 或 as 引导。两者之间的区别在于：
1. which 引导的定语从句只能位于主句后，但as引导的从句可位于主句前、中、后。如：
David, as you know, is a photograopher. 戴维是个摄影师，你是知道的。(不可用which)
Li Ming is late, as is often the case. 李明迟到了，这事是经常发生的。(不可用which)
2. as在定语从句中作主语时，从句谓语通常要有be或别的系动词，但which不受此限制。如：
Li Ming was late, which (=and this) made Mr. Zhang very angry. 李明迟到了，这件事使张先生非常生气。(不可用as)
3. as 引导的定语从句只表示一个众所周知或意

料之中的事，但 which 不受此限制。如：
He married her, which was unexpected. 他和她结婚了，这是出乎意料的。(不可用as)
4. as有“正如”之意，而which则意为“这(件事，一点)”，可用and this/that/it来代替。
5. as多用于固定搭配中：as is often the case(这是常有的事)，as was expected(不出所料)，as often happens(正如经常发生的那样)，as is known to all(众所周知)，as has been said before(如上所述)，as is mentioned above(正如上面所提到的)等。
注：which 代表整个句子，还可用于in which case, at which point, on which occasion等。如：I may have to work late, in which case I’ll telephone you. 我可能得晚点下班，那样我会给你打电话的。