代词之中，最为令人头痛的，或许莫过于关系代词了。咋一听，是不是有点迷糊：啥叫关系代词?先看个例子：The girl who is reading is my sister.在这个句中，who就是关系代词，代指先行词the girl。哦，宝宝们恍然大悟：原来代指定语从句中先行词的代词，就叫关系代词!!!
英语中，关系代词主要有that, which, who, whom, whose, of which, as等。
关系代词不仅代指先行词，还在定语从句中担当主语、宾语、表语和定语。如：
Lily likes music that is quiet and gentle.玛丽喜欢轻柔的音乐。(that作主语)
The film (which) we saw yesterday was wonderful.我们昨天看的那部电影很好看。(which作宾语)
He is no longer the man (that) he was ten years ago.他不再是十年前的那个他了。(that作表语)
The dress whose color is red is bought by my mom.红色那件裙子是我妈妈买的。(whose作定语)
这些关系代词有主格，宾格和属格之分，以及指人与指物之分。如表所示：
1.that可用于指人或物。在从句中作主语或宾语，作主语时不可省略，作宾语可省略。例如：
I like books that are interesting.我喜欢有趣的书。(that作主语)
The book (that) I put on the table is Tom’s.我放在桌子上的那本书是汤姆的。(that作宾语)
2.which用于指物，在句中作主语或宾语，作主语不可省略，作宾语可省略。另外，which还可以引导非限制性定语从句。例如：
The book which is on the desk seems very interesting.桌上那本书貌似很有趣。(which作主语)
The book (which) we saw last night was wonderful.我们昨天晚上看的那本书很不错。(作宾语)
He said the book was so easy, which was not true.他说这本书很简单，那是不对的。(which引导非限制性定语从句)
3.who/whom用于指人，who用作主语，whom用作宾语。在口语中，有时可用who代替whom，也可省略。例如：
The girl who wears red is from England.穿红色衣服的那个女孩是英国人。(who作主语)
The man (whom) Lucy is talking to is my teacher.正在与露西谈话的那个人是我的老师.(whom作宾语)
4.whose用作关系代词时，表示“……的……”：
The students whose names were called stood up.被叫到名字的学生站了起来。
The house whose windows are broken is very old.窗户坏掉的那个房子很旧。
5.特殊关系代词but，as，than
(1)but既可指人或物，常在定语从句中充当主语。but本身含"否定"的意思，其作用相当于"that /which /who...not"。它前面的主句通常有"否定"的词。but引导的肯定定语从句和否定的主句连用，达到强烈肯定的修饰效果。例如：
There is no mother but loves her own children.(=There is no mother that/who does not love her own children.)没有不爱自己孩子的母亲。
(2)as可指人或物，常在限制性定语从句中充当主、宾、表语。
as引导的限制性定语从句的先行词前面常有as /such /the same等修饰词。例如：
He is as brave a man as ever berthed.他是世界上最勇敢的人。
as作关系代词，还可用来引导非限制性定语从句，as引导的定语从句可放在句首、句中或句末。例如：
As we all know, English is very important now.
English, as we all know, is very important now.
English is very important now, as we all know.
(3)than可指人或物。than前通常有表示比较的词。例如：
Fewer guests than we had expected came to the party.来参加晚会的客人比我们预料的还要少。
