我们都知道，which和as都可以引导非限制性定语从句，有时可以互换使用。但两者的用法又有所不同。何时用which，何时又必须用as?是不是有些傻傻分不清?!来来来，今天就把这个问题一次拎清~~~
(1)位置上，as可放在句首、句末，也可插入主句中;which一般放在主句之后(有时也可用as)：
As we all know, reading is very important.
Reading is very important, which we have already known when we were very young.
(2)意义上，as有正如、按照、正像之意，一般用在肯定句中;which可用于含肯、否定意义的句子中：
He failed in the exam again, as was expected.
He failed in the exam again, which was unexpected.
(3)当定语从句所修饰的主句内容是谓语动词的对象时，常用as，如：
Jack was admitted into the university, as we had expected.
(4)固定结构，用as：
as has been said before as often happens
as is well known as we all can see
the same...as such...as
(5)当关系代词代表主句引起的结果，又做主谓宾补(SVOC)结构中的主语时，多用which：
He passed the exam, which made him delighted.
(6)在从句中作介词宾语时，用which，如：
They went to the zoo, after which they saw a film.
(7)在从句中代表主句中的谓语的整个概念时，常用which，如：
He can write a letter in English, which I cannot.
(8)指代主句中某一名词时，多用which，如：
Beijing, which he was born, is our capital.
(9)代替主句中的形容词时, 常用which，如：
Lily thought me clever, which she herself was.
在定语从句中，相比于which VS that，who VS that，我们对which与as两者区别的关注度要少很多，在遣词造句或是口语表达中，也常常是“跟着感觉走”。