定语从句 （一）定语从句一般由关系代词和关系副词引导
1、关系代词：who,whom,whose,which,that,as
2、关系副词：when,where,why
P.S ：关系代词和关系副词必须位于从句之首，主句先行词之后，起着连接先行词和从句的作用，同时在从句中又充当句子成分。
e.g.She is the girl who sings best of all.（关系代词who在从句中作主语）
The comrade with whom I came knows French.（whom在从句中作介词with的宾语）
3、关系代词和关系副词的用法：
1）当先行词为人时用who作主语，whom作宾语；
2）当先行词为物或整个句子时用which，可作主语或宾语；
3）先行词为人、物时用that，可作主语或宾语；
4）whose用作定语，可指人或物；
5）关系副词when（指时间，在定语从句中作时间状语，where（指地点，在定语从句中作地点状语），why（指原因，在定语从句中作原因状语）。
（二）限制性定语从句与非限制性定语从句
1、限制性定语从句：从句与主句关系密切，去掉从句，主句意义不完整，甚至不合逻辑。
e.g.I was the only person in our office who was invited.（去掉定语从句，意思就不完整）
2、非限制性定语从句：从句对先行词关系不密切，去掉定从句，意思仍然完整。形式上用逗号隔开，不能that用引导。
e.g.Tom’s father,who is over sixty,still works hard day and night.（who引导非限制性定语从句，整个句子可分成两句来翻译）
（三）使用定语从句时特别注意的几个问题
1、that与which的区别。
1）用that而不用which的情况：
①先行词为不定代词all,anything,nothing,…；
②先行词有最高级修饰，有序数词修饰；
③先行词有only,very,any等词修饰；
④先行词既有人又有物时。
e.g.There is nothing that can prevent him from doing it.没有什么能阻止他不干那件事。
The first place that they visited in Guilin was Elephant Trunk Hill.在桂林他们所参观的第一个地方是象鼻山。
This is the best film that I have ever seen.这部电影是我看过的最好的一部。
Mr Smith is the only foreigner that he knows.史密斯先生是他认识的唯一的外国人。
2）用which而不用that的情况：
①引导非限制性定语从句；
②代表整个主句的意思；
③介词+关系代词。
e.g.He had failed in the maths exam,which made his father very angry.他数学考试没有及格，这使他的父亲很生气。
This is the room in which my father lived last year.这是父亲去年居住过的房子。
3）as引导定语从句时的用法
①as引导限制性定语从句通常用于thesame…as,such…as结构中。
e.g.I want the same shirt as myfriend’s.我要一件跟我朋友一样的衬衫。
Such machines as are used in our workshop are made in China.我们车间使用的这种机器是中国制造的。
②as引导非限制性定语从句既可放在主句之前，也可放在主句之后，用来修饰整个句子。通常用下列句型：
as is known to all,as is said,as is reported,
as is announced,as we all know,as I expect等。
e.g.As I expected,he got the first place again in this mid-term examination.正如我所预料的那样，他在这次期中考试中又获得了第一名。
3）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whom，且不能省略；介词在句尾，关系代词可有which,that,whom,口语中也可用who，且可省略。
e.g.The man(whom/who/that)you were talking about has come to school.
参考资料：
http://baike.baidu.com/view/56536.html?wtp=tt