1.当 as 引导限制性定语从句时，常构成 the same…as; such as; such…as; as many…as; as much…as; as…as; not so/as…as等固定结构，在这种结构中是不能使用which的。例如：

I read the same book as you read. 我读的书与你读的一样。

This dictionary is not such as I expect. 这并不是我期望得到的词典。

He told me to read as many books as I could. 他告诉我尽可能多读些书。

▲ the same...as中的as也可用that，但意义不同。试比较：

① This is the same bag as I lost. (与我丢的包相同 / 一样)

② This is the same bag that I lost. (就是我丢的那个包)

2.在引导非限制性定语从句时，as和which都可指代主句的部分或整句的内容，在从句中做主语，宾语或表语。但要注意以下几点：

① as引导非限制性定语可以放在主句前面、中间或末尾。例如：

As is known to all, the earth is round. 正如大家都知道的那样，地球是圆的。 (as指代主句，作主语，置于句首)

John, as we had expected, passed the exam. 正如我们所预料的，约翰通过了考试。(as 指代主句，作宾语，置于句中)

Mary is late again, as is often the case. 玛丽又迟到了，这是常有的事。(as 指代主句，作主语，置于句尾)

▲as anybody can see (正如任何人都能看到的)；as we all know (正如我们大家所知道的); as is well-known (众所周知); as we had expected (正如我们所预料的); as often happens (正如经常发生的); as has been said before (如上所述); as is often the case (这是常有的事)......等为常用结构，此时不用which。

② 当定语从句位于句尾表“正如......; 就像......”时要用as，而which则意为“这件事；这一点” (相当于and this / that / it)，它们所表达的意义是有差异的。试比较：

Everyone likes Tom, as you can see. 正如你所看到的，大家都喜欢汤姆。

Everyone likes Tom, which you can see. 大家都喜欢汤姆，这一点你是能够看得出来的。

③ 当定语从句位于句尾，谓语动词是be或其它的连系动词 (如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，要用which作主语。例如：

He failed in the driving test, which / as was natural. 他驾考挂科了，这是很自然的事。

Moyan won the Nobel Prize in literature, which made him illustrious. 莫言获得了诺贝尔文学奖，这使得他名扬天下。

④ 当定语从句位于句中指代某个名词或代词而不指代整个句子时，不用as。例如：

The flat, which I bought last year, has not been decorated. 这个公寓是我去年买的，还没装修呢。

下面句子中的 as 一般认为是关系代词：

As I explained on the phone, your request will be considered at the next meeting. 正如我在电话里解释的那样，下次开会将考虑你的请求。

As Napoleon once said, attack is the best method of defence. 拿破仑曾经说过，进攻是最好的防御。

As is often the case with children, Amy was completely better by the time the doctor arrived. 就像经常发生在儿童身上的情况一样，医生赶到时，艾美的病情已彻底好转了。

但查了一下牛津、朗文、剑桥等权威词典，却都认为它是conjunction（连词），不明白这是什么原因，求解！

(1)关系代词也是属于代词的种类范畴。(关系代词起着承上启下作用，代替先行词同时在从句充当成分）

(2)连词分为：并列连词和从属连词，引导并列句为并列连词，引导主从复合句叫从属连词。

(1)引导定语从句“所有连词”称作为“关系词”

关系词分为以下2类

A:关系代词：（在定语从句充当主语，宾语，表语，定语）

who, that, which, as, whose

B:关系副词：（只能在定语从句作状语成分）

when, where, why,

(2)引导名词性从句“所有连词"称作为连接代词和连接副词或者其他连词。

A:连接代词：

what;which;who;whom;whose

Whatever;whichever;whoever;whomever;whosever

B:连接副词

Where;when;why;how

wherever;wheneven;however

C:其他连词

because, if, whether, that 

(3)引导9大状语从句所有连词称作为“从属连词”。

A: when, while, as, as soon as, until, 

B: where, wherever,

C: if, unless

D: because, as, since, 

E: so that, so...that.

F: in order that, so that

G: though, although, as 

H: than,  as ..as

J: as, as if, as though