英语：定语从句中什么时候用As作为先行

引导状语从句的连词as可以用作关系代词，引导定语从句，因而被称作准关系代词。
1. 准关系代词as引导限制定语从句，用于以下3个结构中，as在从铅备句中作主语、宾语或表语：
a. so / such…as
Such people as were mentioned by him were honest. (as作主语)
I’ve never seen such a clever man as he is. (as作表语)
Last term our maths teacher set so difficult an (= such a difficult) examination problem as none of us worked out.
b. the same…as
Modern statemen are often faced with the same problem as defeated the ancient Romans. (as 作宾语)
I like the same book as you do. (as 作宾语)
c. as…as
As many children as came were given some cakes. (as 作主语)
He is as diligent a man as ever lived. (as 作主语)
补充：
a. such…as 与 such…that的区别
such…as 引导定语从句；such…that引导结果状语从句。
They talked in such simple English as children could understand.
他们用孩子们能听懂的简单的英语交谈。
They talked in such simple English that children could understand it.
他们用十分简单的英语交谈，连孩子们都能听懂。
I will provide you with such things as you may need.
我要供给你一些你可以用得着的东西。
He shut the window with such a force that the glass broke.
他如此用力地关门，结果玻璃碎了。
b. the same…as与the same…that的区别：
the same…as指两物相似，the same..描述的是同一物。
This is the same bag as I lost yesterday.
这个书包和我昨天丢失的（书包）相似。（相似物）
This is the same bag that I lost yesterday.
这正是我昨天丢失的那个书包。（同一物）
2. 准关系代词as引导非限制定语从句。
a. as引导非限制性定语从句，只代表主语或主句的一部分所表达的意思(不能指代某个名词或代词)，可以放在主句前，后或中间，游备在从句中作主语或宾语、例如：
As is known to all, he is the best student in our class. (as指代整个句子，作主语，置于句首)
He opposed the idea, as could be expected. (as 指代整个句子，作主语，置于句末)
As we all know, he studies very hard. (as 指代整个句子，作宾语，置于句首)
Taiwan is, as you know, an inseparable part of China. (as指代整个句子，作宾语，置于句中)
常用的这类句式有：
as is said above 综上所神激毁述 - as is known to all 众所周知
as is often the case 通常如此 - as is reported in the newspaper 如报纸所报导的
b. 主句具有否定意义，as引导的定语从句位于主句之后时，as指代得主句不包括否定意义，并可用but取代。例如：
Whales are now fish, as some people think / but some people think they are.
I am no orator, as Brutus is.我不善言词,而勃鲁特斯却是个雄辩家.
Spiders are notinsects,as many people think, nor even near1y related to them.
蜘蛛不是昆虫,尽管许多人认为它是昆虫,它甚至与昆虫几乎没有关系.
但as引导的定语从句谓语主句前时，as指代的主句仍包括否定意义。
As has been said above, grammar is not a set of dead rules.
正如前面所说的，语法不是一套死条纹。
3. 在非限制性定语从句中，which和as引导定语从句时的区别：
a. which引导的从句不能放在主句前，而as引导的从句则既可以放在主句前，也可以放在主句后。
b. 用能做关系代词，as和which都可以指代一个句子，但as有“正如”、“就像”之意，which则没有此含义。
He is a foreigner, as I know from his accent. (as指代一个句子，意为“正如”、“就像”)
He has to work on Sunday, which he doesn’t like. (which指代一个句子，没有“正如”、“就像”)
c. 当从句的谓语动词是be或相当于联系动词的seem, become等时，可用as或which作主语。但当从句的谓语是行为动词时，只能用which作主语。
4. as 可用作连词，引导状语从句，构成省略形式“as + 过去分词”这一特殊结构，译成汉语时作用如定语。例如：
The bridge as shown in the picture is really wonderful. (as shown = as it is shown 可译为“图中的桥”)
See the answers as given at the end of this book. (as given = as it is shown 可译为“书后给出的答案”)
English as spoken is Australia is slightly different from British English.
(as spoken = as it is spoken 可译为“在澳大利亚讲得英语”)
请看下列的“as + 过去分词”结构用于修饰前面的名词：
The study as described in the article… 如文章中描述的那种学习
The program as conducted by… 由…指挥的（指导的）节目
The art as applied to medicine… 应用于医学的艺术
The products as seen on TV…电视上看到得产品