as引导限制性定语从句
as引导的限制性定语从句，主要出现在一些固定的结构中如such...as， the same...as等，此外它也可以单独使用，as在限制性定语从句中可用作主语、宾语和表语
such(...)as结构
这种结构中的such既可用作形容词，作先行词的定语，即构成\"such+名词+as”结构;
也可用作代词，充当as的先行词，即构成such as结构。 例如:
Transistors have such advantages as are described in the book. 晶体管具有像本书所描述的那些优点。 (as在从句中作主语)
She is not such a careless student you think she is. 她不是你所认为的那样粗心的学生。
(as在从句中作表语)
Such symbols as we often use in mathematics are Greek letters. 我们在数字中常用的那些符号是希腊字母。 (as在从句中作宾语)
The instrument is not such as I saw on the exhibition. 这台仪器不是我在展览会上看到的那种。
(as在从句中作宾语，such用作代词充当as的先行词)
因作形容词的such有时可以置于先行词之后，使其和as紧紧连在一起，形成\"such as”结构。实际上是such作前位定语的一种变体。例如:
例如:
The library was full of story-books such as boys love. (=The library was full of such story-books as boys love. )
图书馆堆满了孩子们所喜爱的那种故事书。
the same (... ) as 结构
这种结构基本上和such(...)as相同。same是形容词，修饰先行词，也可以是代词，充当as的先行词。same总是和定冠词用在一起。 例如:
It is said that the moon is made of the same kind of matter as exists on the earth. 据说,月球是由与地球上存在的相同物质组成的。(as在从句中作主语) Although he tried another method, he arrived at the same conclusion as we had. 尽管他采用了另一方法,但是他得出的结论和我们的一样。 (as在从句中作宾语)
He is not the same man as he was before. 他不是从前那样的人了。 (as在从句中作表语)
It was, of course, just the same with the official as it had been with the minister. 当然,这位官员所遇到的情况和那位大臣所遇到的完全一样。 (the same作代词，作as的先行词,as在从句中作表语)
当先行词受same修饰时，关系代词亦可用that，但必须注意它们的区别。用that一般表示从句的内容与主句所表示的是“同的”;用as则通常表示“与…相似的”意义，从句表示的内容与主句的内容相似。 试比较:
This is the same (-the very ) bag that I lost yesterday.
这是我 昨天丟失的那个书包
This is the same bag as I lost yesterday. (This bag is like the one that lost yesterday.) 这个书包和我昨天丟失的书包相似 。
as many as 和 as much as 结构
在这种结构中，第一个as是副词，修饰many或much， many或much分别接复数可数名词和不可数名词。第二个as是关系代词。 例如:
There are as many books as are needed.
凡是需要的书都有了 ( as 在从句中作主语 )
As many books as are on the shelf can be taken as teaching materials. 凡在书架上的书都可用作教材。 (as在从句作主语)
Take as much paper as you need, but not more than is necessary. 按需要取纸,但不要多拿。
(as在从句中作宾语)
这种结构中的many和much也可用作代词，充当其后as的先行词。 例如:
There is plenty of sulfuric acid here, you may take as much as you want. 这儿硫酸很多,你可以按需要尽量取用。
as... as 和 so... as 结构
这种结构中的第一个as和so均为副词，其后紧跟形容词，形容词后通常再接带不定冠词a(an)的单数名词。 例如:
It was as pleasant day as I have ever spent. 那是我度过的最愉快的一天。 (as在从句中作宾语)
Rosey is as good a little creature as can be.
罗西是一个再好不过的人了。
(as在从句中作表语,从句中的主语anybody被省略)
We witnessed so great a parade as nobody ever dreamt of in preliberation days. 我们亲眼看到过在解放前做梦也想不到的那样大规模的游行。 (as在从句中作宾语)
这种结构可以和\"such.\"结构换用但必须注意不定冠词as，的位置。 例如:
Here is so big stone as no man can lift.
(=Here is such a big store as no man can lift. ) 这儿有一块没人能拿得起的大石头
as单独使用所引导的定语从句
as亦可单独引出限制性定语从句只是在含义上与那些固定，结构有所不同罢了。 例如:
She fought against it, like a brave woman as she was.
像她过去那样她勇敢地挺身而出极力反对这件事。 (as在从句中作表语)
From every corner of the island little people came to see \"Man-mountain\" as they called Gulliver. 小人們从海岛的四面八方来看这位被称为\"人山”的格利佛。 (as在从句中作宾语)
as引导的非限制性定语从句
在as引导的非限制性定语从句中，as常用以代替整个主句或者主句的一部分，as在从句中可用作主语、宾语、表语等。从句的位置比较灵活，可以位于句首、句中或句末。
例如:
As is mentioned above, a strengthened field is created where the lines of force in the two magnetic fields are parallel and have the same direction.
如上所述，两个磁场的磁力线相互平行且方向相同时，就形成了一个加强的磁场。 (as在从句中作主语)
Metals have many good properties, as has been stated before.
金属具有许多良好的性能,这在前面已经讲了。(as在从句中作主语)
Carbon monoxide is poisonous, as are the gases we are going to discuss in the next chapter.
像下一章中要讨论的气体一样,一氧化碳也是有毒的。 (as代替主句中的表语poisonous,并在从句中作表语) Gunpowder, as we all know, was invented in ancient China 大家知道火药是中国古代发明的。 (as代替整个主句并在从句中作宾语)
Gasoline is combustible, as are all of the liquids listed in the following section. 汽油是易燃的,下一节里所列的那些液体也都是易燃的。 (as代替主句的表语部分,并在从句中作表语) 在as引导的非限制性定语从句中，当as作主语，谓语为及物动词的被动语态时，常出现省略助动词的现象，即常使用简略被动式。这种形式也可以灵活地放在句首、句中或句末。 例如:
The cost of manufacture increases rapidly as the tolerances are reduced, as indicated by the typical curve of the figure.
如图中的典型曲线所示,生产成本随公差的减少而迅速增加。
As mentioned above, an object has positive charge when it has lost electrons. 如上所述，物体失去电子时带正电荷。
Levers and pulleys, as shown in the experiment, change motion from one direction to another. 如实验所示杠杆和滑轮可改变运动方向。
as与which引导的非限制性定语从句的区别 位置
as引导的非限制性定语从句比较灵活可以放在句首、句中或句末(见以上例句);而which引导的非限制性定语从句只能放在主句之后。 例如:
Metals are good conductors of electricity, which means that they have more free electrons than other substances.
金属良导体,这意味着它们比其他物质有更多的自由电子。
Light travels in straight lines, which explains why shadows are formed when it goes past an object. 光以直线传播这说明了光经过物体时阴影形成的原因。 成分
as在非限制性定语从句中只能作主语、动词宾语和表语(见以上例句)而which，除了能作主语、动词宾语和表语外，还能作定语和介词宾语。 例如:
To find the pressure we divide the force by the area on which it press, which gives us the force per unit area.
要求压强,则把力除以它所作用的面积,从而得出单位面积的力。 (第一个which作介词on的宾语,第二个作主语) He admires Mrs Brown, which I find strange.
他钦佩布朗夫我对此感到奇怪。 (which作主语)
The resistivity of most insulators decreases with an increase in temperature, for which reason the temperatures in insulated conductors must be kept reasonably low. 大多数绝缘体的电阻率随温度升高而减少,因此,被绝缘的导体的温度必须保持适当低。
(which作定语)
The steel must be held at furnace temperature for sufficient time to dissolve the carbides in the austenite, after which, the steel can be cooled.
钢件一定要在炉温下保持足够长的时间,以使碳化物溶解到奥氏体中,然后才可使钢体冷却。
(which作介词after宾语)
谓语动词是及物动词时
当非限制性定语从句中谓语动词是及物动词时，as与which虽都可作主语和宾语，但两者有区别。当从句所修饰的主句的内容是从句谓语动词动作的承受者或
者说是从句所表达的内容的结果时，通常用as；当其所修饰的主句所述的内容是从句谓语动词动作的发出者或者说是从句所表达的内容的起因时，则通常用which。 例如:
He saw the girl, which delighted him. 他见到了这个姑娘,这使他很高兴。 He saw the girl, as(*which) he had hoped. 他见到了这个姑娘，这是他所希望的。
但是当which本身带有补足语时，从句谓语动词可以是被动语态这时which就成了从句谓语动词动作的对象，但起着其补足语的逻辑主语的作用。 例如:
Salt is crystallized from sea water, which is known to be a physical change. 盐是从海水中结晶出来的,这被认为是一种物理变化。
All forces occur in pair, which may conveniently be spoken of as action and reaction. 所有的力都是成对出现的,这可以很方便地谈到作用和反作用的问题。
当然无论语法规则还是表达习惯，总会有例外。我们偶尔还是可以见到关系代词which在非限制性定语从句中不带补足语而直接作及物动词的动作对象的情况。 例如:
Light from common sources is a mixture of light of many different colours or frequencies, which can be proved by breaking up a beam of light with a prism. 来自普通光源的光是许多不同颜色或频率的混合光,这可用棱镜来分解光加以证实。
Water consists of molecules composed of two atoms of hydrogen and one atom of oxygen, which we describe by the formula H2O.
水分子由两个氢原子和一个氧原子构成,因此我们用H20表示它的分子式。
谓语动词是连系动词be时
当非限制性定语从句的谓语动词是连系动词be时，虽然which和as都可作从句中的表语或主语，但两者也有一定的区别只有在不损害as在状语从句中所具有的那种“正如\"，\"像那样”的原来语义时才可以任意选用as或which来引导从句，否则就只能用which来引导。 例如:
He married her, as/which was natural. 他与她结了婚,这是很自然的。
He is again deeply absorbed in his work, as/which he often is.
他又埋头工作,他常常是这样的。
上面两句中as与which均可使用，因为句子前后意思是自然连贯的下面两句由于从句的意思的转折，就只能用which引导了。 例如:
He married her, which was disgraceful.
他与她结了婚,这是不光彩的事。
She was very patient towards the children, which her husband seldom was. 她对孩子们很耐心,她丈夫却很少这样。
这种情况也还有个习惯搭配问题 。 如 \" as is often the case \" , \"as is usual\is very kind of him\等中的 as 和 which 则一般不可随意换用 。
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