在英语语法学习中，我们知道as和what都可以引导定语从句，这两者有什么区别呢？

1 、 关系代词在非限定性定语从句中代替主句中某一个名词或词组，并对该名词或词组进行补充或说明时，只能用Which.

The book, which I bought yesterday, is very interesting.

Last night, I saw a film, which was about Long March.

2、关系词在非限定性定语从句中代表主句中谓语部分的整个概念的只能用which.

He can write a letter in English, which I can not.

I asked him to lend me his bike, which he did.

3、关系词在非限制性定从中代表主句所表达的整个内容，并当关系词等于and this时，只能用which.

The stems of bamboo are hollow, which makes them very light. It was raining hard, which kept us indoors.

4、 关系词在非限制性定从中代表主句所表达的整个内容,并关系词等于which fact时，只能用as

This elephant is like a snake, as anybody can see. He came very early this morning, as was usual.

5、关系代词所引导的非限定性定从放在主句之前时，只能用as.

As we all know, Taiwan is a part of China.

6、非限定性定从的谓语为say, know, imagine, announce, show, report, expect, explain, point out等动词的被动形式时，只能用as.

He is a famous scientist, as is well known.

As is announced in today’s newspaper, American president will pay a visit to China.

7、 关系词在非限定性定从中代表主句中的表语意思，并在从句中作表语时，只能用as.

This question is very simple, as indeed it is.

This question is quite easy, as are all of the questions in the following.

8、如果定语从句的内容对主句有消极作用 或含义不一致，则从句用which引导，如果语义一致时，则常用as引导。

She did nothing but sit there, which showed her laziness.

Our team lost the game at last, which made us disappointed.

9、从句中的谓语是复合结构时，用which引导。

Bob always tells a lie, which his parents find strange.

10、 从句是否定结构时，多用which.

Mr Smith often praised his student Rose in public, which she didn’t like all.

11、在从句中作主语时，as多用于被动语态，which多用于主动语态。

He has been late again, as is expected.