Question

有关定语从句as的用法

Answer 1

由as, which 引导的非限定性定语从句，as和which可代整个主句，which相当于and this或and that。1)As一般放在句首，which在句中。2) as也可在句中往往和see,know,expect等词连用。
a. He seemed not to have grasped what I meant, which/and this greatly upsets me.
b. The sun heats the earth, which/and this is very important to us.
c. As was expected, the beautiful girl married a handsome boy.
d. As we all know, smoking is harmful to one's health.
e.He must be from Africa, as can be seen from his skin.
定语从句中若先行词前有the same, such作修饰语，通常用as作关系代词。
He used such expressions as he could find in the texts.
They stayed for the night in the same room as they had once rented.
such…as和such..that
such…as定语从句, as在从句中可以作主语,宾语,表语；such…that引导结果状语从句，that不充当任何成分，只起到连接作用，如：
They talked in such simple language as children could understand.
They talked in such simple language that children could understand it.
the same…as和the same…that
两者都可以引导定语从句，但the same…as指两件事物,而the same…that指同一件事物：
He is using the same pen as I did 10 years ago.
他用的这支钢笔和我十年前用的那支一个样。
He is using the same pen that I did 10 years ago.
他用的这支钢笔就是我十年前用的那支

Answer 2

as 用作关系代词，主要用法有：
(1) 用在 such, same, as 等之后，引导限制性定语从句。如：
I live in the same place as Tom does. 我和汤姆住在同处。
He’s not such a fool as he looks. 他并不像看上去的那样愚蠢。
He smokes as expensive a cigarette as he can afford. 他抽一种他买得起的最贵的烟。
(2) 单独用作关系代词，引导非限制性定语从句，可放在主句之前、之后或中间，且主从句之间一般要用逗号隔开。如：
As was expected, he succeeded at last. 正如我们所料，他终于成功了。
I had coffee after dinner, as is my usual practice. 我饭后喝咖啡，这是我的习惯。
Freddie, as might be expected, was attending the conference. 弗雷迪在参加会议，这是可以预料到的。
(3) as与which的区别
① 引导限制性定语从句的区别：as用作关系代词引导限制性定语从句时，它通常只用于 such, same, as 等之后，而which 用作关系代词引导限制性定语从句时，则用于除 such, same, as 之外的其他结构。
② 引导非限制性定语从句的区别：which引导非限制性定语从句时，通常只能位于主句后，而as引导的非限制性定语从句位于主句之前、之后或中间。
根据从句谓语动词的特点来看，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)
根据句意的逻辑性来看，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, as [which] was natural. 他和她结婚了，这是很自然的。
He married her, which was unexpected. 他和她结婚了，这是出乎意料的。（不可用as）
摘自《英语常用词多用途词典》（金盾出版社）