从句中as与which的区别

一般可以互换。但注意：
1.若做从句主语，当谓语是be时，可互换。
2.若做从句主语，当谓语是非be时，只能用which.
3.若做从句宾语或表语，则无论如何又可随意互换了。

as引导非限制性定语从句时与which的区别

①当主句和从句语义一致时，用as；反之，用which来引导非限制性定语从句。e.g.He made along speech,as we expected.

He made along speech,which was unexpected.

②当非限制定语从句为否定时，常用which引导。

e.g.Tom drinks a lot everyday,which his wife doesn’t like at all.

2.关系代词在从句中作主语时，从句的谓语动词单数还是用复数应由先行词决定。e.g.The man who lives downstairs speaks English fluently.住在楼下的那个人英语说得很流利。 The students who are in GradeThree are going to climb the hill tomorrow.

3.定语从句有时不直接紧靠先行词，中间由一个定语、状语或谓语隔开。e.g.There is an expression in his eyes that I can’t understand.

4.引导定语从句的关系副词有时可以用“介词+which”来代替。

e.g.October 1,1949 was the day on which(=when)the People’s Republic of China was founded.

5.当定语从句中谓语动词是带介词或副词的固定短语动词时，短语动词的各个固定部分不要拆开。

e.g.The sick man whom she is looking after is her father.

6.介词在关系代词前，只能用which和who