在限制性定语从句中，as 与 which 一般是不会被误用的。较难掌握的是在非限制性定语从句中该如何正确地使用它们。

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 Icould. 他告诉我尽可能多读些书。

注意: the same ... as 中的 as 也可用 that，但意义不同。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)

注意: 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，此时不能用 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，只能用 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，不能用 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。