英语中的定语从句是难点，也是考试热点。
原文标题：as(代词)引导定语从句和as(连词)引导评述性状语从句
分两大类情况讲解：
下边详细讲解。
（一）as(代词)引导限制性定语从句
1. 当先行词是“such/the same 名词”时，用as引导定语从句。即，such n. as(代词)，the same n. as(代词)。
例句（as指代such n.，作主语/宾语/表语）
●We invited all such people as were likely to come.（as在从句中作主语。as=all such people，即，All such people were likely to come.）
●I have never heard such stories as he tells. 我从未听说过他讲的这类故事。（as在从句中作宾语。as=such stories，即，He tells such stories。）
●I’ve never seen such a fool as he is.（as在从句中作表语。as=such a fool，即，He is such a fool.）。
例句（as指代the same n.，作主语/宾语/表语）
●Is this the same mosquito as bit you just now？（as作主语。The same mosquito (=as) bit you.）
●This is the same knife as I lost yesterday.（as作宾语。I lost the same knife (=as)yesterday.）
●She is just the same person as she used to be.（as作表语。She used to be the same person (=as).）
2.as与that的区别。
①such…as(代词)引导定语从句，such…that(连词)引导结果状语从句。如，
●He is such a good boy as everyone likes.他是个人人都喜欢的好男孩。
（从句缺成分（缺宾语），因此as是代词（作成分））
●He is such a good boy that everyone likes him.他是个好男孩，以至于人人都喜欢他。
（从句不缺成分，因此that是连词（不作成分））
②the same…as(代词)…，表示两物相似；the same…that(代词)…,表示两物同一物。如，
●This is the same pen as I used yesterday.这是和我昨天用的一样的钢笔。
●This is the same pen that I used yesterday.这是我昨天用过的那支钢笔。
注意：the same …，可以与代词as/that连用，也可以与副词when/where等连用。如，
●This is the same place where we had the party that day.（where作状语）
（二）as(连词)引导评述性状语从句
评述性状语从句，表示说话人的看法、态度、解释或评论。as的意思是“正如/像”。
学习这块内容，关键在于区别as与which。本书认为，as（连词）引导评述性状语从句，which（代词）引导非限制性定语从句。所有的能换用/不能换用的情况，都可以用这点区别推导出来。下文中，1.讲不能换用的情况；2.讲能换用的情况。1和4是重点内容，2和3仅供了解。
1. as (连词)引导评述性状语从句，which (代词)引导非限制定语从句。
（1）引导词词性不同。as是连词，which是代词。由此可以推导出：
（2）从句类型不同。as引导的是状语从句，which引导的是定语从句。由此可以推导出：
2. as与which可以换用的三种情况，纯属巧合。
为何说纯属巧合呢？因为，as引导的是状语从句，它可以省略无意义的it（作主/宾语时）或承前省略（作表语时）。在省略掉这些词之后，凑巧可以把as换作which。
（1）which在“主系表”中作主语时，有时可以替换为as。试对比：
●He married her, which was natural.（不能插入it。为何此处不能插入it呢？因为，which是代词，在从句中作成分，如果再插入it，则重复了。）
●He married her, as (it) was natural.（有无it都可以。为何此处有无it都可以呢？因为，as是连词，as在从句中不作成分，所以，插入it之后，也不重复。又因为as引导的是状语从句，而状语从句有时可以省略无意义的主语it。）
●She has been absent again, which is expected/(which expected×).（“which be v-ed”时，不能插入it，也不能省略be。）
●She has been absent again, as (it) is expected/as expected.（“as be v-ed”时，可以插入it，也可以省略be。）
注意，which在“主谓宾”中作主语时，不能替换为as。如，He saw the girl, which/(as×)delighted him.
Tom was late for school again, which/(as×) made his teacher angry.（这两个句子为何不能看作as从句省略it呢？因为，在状语从句中，一般在“主系表”情况下，才可以省略it，而以上两个句子都是“主谓宾”。）
（2）which作宾语时，有时可以替换为as。试对比：
●He is from the south, which we can know from his accent.（不能插入it）
●He is from the south, as we can know (it) from his accent.（有无it都可以）
（3）which作表语时，有时可以替换为as。试对比：
●He seemed a foreigner, which in fact he was.（不能插入a foreigner）
●He seemed a foreigner, as in fact he was (a foreigner).（有无a foreigner都可以。因为，在状语从句中，承前省略了a foreigner。）
总之，as是连词，which是代词，两者截然不同。二者能换用的情况，纯属巧合，并不能说明它们词性相同。打个比方，一个马群和一个牛群，偶尔在同一地方吃草，但是，马还是马牛还是牛。
3.常见的as引导评述性状语从句。见下表，
注：关于状语从句中的省略，可以参看本章第七节以及第十二章。
4. 注意as/it/what的区别。如，
●What is known to us all is that China has the largest population in the world.
（what引导主语从句，that引导表语从句）
●It is known to us all that China has the largest population in the world.
（it作形式主语，真实主语是后边的that从句）
●As is known to us all, China has the largest population in the world.
（as引导评述性状语从句）
分组对比练习
题组28（as与which）
1. _____ is often the case, we have worked out the production plan.（2004,江苏）
A．Which B．When C．What D．As
分析：as引导评述性状语从句，可以放在主句前，也可以放在主句后。which引导定语从句，不能放在主句前，只能放在主句后。选D。
2. A lot of language learning, ____has been discovered, is happening in the first year of life, so parents should talk much to their children during that period .（2012,安徽） A．as B．it C．which D．this
句意：正如研究发现，很多语言学习发生在出生后的第一年，所以，在那个阶段，父母应该多和孩子说话。选A。
3. The owner of the cinema needed to make a lot of improvements and emply more people to keep it running, ____meant spending tens of thousands of pounds.(2006,江苏) A.who B.that C.as D.which
分析：which (代词)在从句中作成分（作主语），as (连词)在从句中不作成分。选D。
4. If the man is only interested in your looks, ____ just shows how shallow he is. (陷阱题)
A. as B. which C. what D. that
分析：容易误认为逗号后是非限制性定语从句，从而误选A或B。其实，注意逗号前是一个条件状语从句，逗号后是该状语从句的主句，that 在此代表前文所述的情况，用作主句的主语。选D。
5. —Who should be responsible for the accident?—The boss, not the workers.They just carried out the order____.（2008,福建） A．as told B．as are told C．as telling D．as they told
分析：as (they are) told，选A。
题组29（as/it/what）
1. _____ is known to everybody, the moon travels round the earth once every month.（全国卷）
A. It B. As C. That D. What 选B。
2. The air quality in the city, ____is shown in the report, has improved over the past two months. (2012,福建) A. that B. it C. as D. what
句意：正如报告中所写的那样，在过去的两个月中，这座城市的空气质量提高了。
分析：把从句放在句首就很明白了: is shown in the report, the air quality in the city has improved over the past two months. “正如在报告中所写的那样”，as指的是下文所提到的话。选C。
3. ____ in the regulations that you should not tell other people the password of your email account.（上海高考题） A. What is required B. What requires is C. It is required D. It requires
句意：规则要求你不应把你的电子邮件的密码告诉别人。选C。
4. ____ was most important to her, she told me, was her family. (2008,山东).
A. It B. This C. What D. As
分析：what是主语从句的引导词，其余都不是。选C。
5. Everyone knows that ____ is dangerous to play with fire，but ____ is difficult is to prevent children from playing with fire. (陷阱题). A. it, it B. what, what C. it, what D. what, it
分析：容易误选A或B。其实，第一空填形式主语it，真正的主语是其后的不定式to play with fire。第二空填 what，what is difficult 是主语从句（注意到what is difficult 后有is，若没有这个is，则第二空也填it）。答案是C。
本文作者：高伟，编著《魔方英语语法》（高中版和教研版）。
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