代詞之中，最為令人頭痛的，或許莫過于關系代詞了。咋一聽，是不是有點迷糊：啥叫關系代詞?先看個例子：The girl who is reading is my sister.在這個句中，who就是關系代詞，代指先行詞the girl。哦，寶寶們恍然大悟：原來代指定語從句中先行詞的代詞，就叫關系代詞!!!
英語中，關系代詞主要有that, which, who, whom, whose, of which, as等。
關系代詞不僅代指先行詞，還在定語從句中擔當主語、賓語、表語和定語。如：
Lily likes music that is quiet and gentle.瑪麗喜歡輕柔的音樂。(that作主語)
The film (which) we saw yesterday was wonderful.我們昨天看的那部電影很好看。(which作賓語)
He is no longer the man (that) he was ten years ago.他不再是十年前的那個他了。(that作表語)
The dress whose color is red is bought by my mom.紅色那件裙子是我媽媽買的。(whose作定語)
這些關系代詞有主格，賓格和屬格之分，以及指人與指物之分。如表所示：
1.that可用于指人或物。在從句中作主語或賓語，作主語時不可省略，作賓語可省略。例如：
I like books that are interesting.我喜歡有趣的書。(that作主語)
The book (that) I put on the table is Tom’s.我放在桌子上的那本書是湯姆的。(that作賓語)
2.which用于指物，在句中作主語或賓語，作主語不可省略，作賓語可省略。另外，which還可以引導非限制性定語從句。例如：
The book which is on the desk seems very interesting.桌上那本書貌似很有趣。(which作主語)
The book (which) we saw last night was wonderful.我們昨天晚上看的那本書很不錯。(作賓語)
He said the book was so easy, which was not true.他說這本書很簡單，那是不對的。(which引導非限制性定語從句)
3.who/whom用于指人，who用作主語，whom用作賓語。在口語中，有時可用who代替whom，也可省略。例如：
The girl who wears red is from England.穿紅色衣服的那個女孩是英國人。(who作主語)
The man (whom) Lucy is talking to is my teacher.正在與露西談話的那個人是我的老師.(whom作賓語)
4.whose用作關系代詞時，表示“……的……”：
The students whose names were called stood up.被叫到名字的學生站了起來。
The house whose windows are broken is very old.窗戶壞掉的那個房子很舊。
5.特殊關系代詞but，as，than
(1)but既可指人或物，常在定語從句中充當主語。but本身含"否定"的意思，其作用相當于"that /which /who...not"。它前面的主句通常有"否定"的詞。but引導的肯定定語從句和否定的主句連用，達到強烈肯定的修飾效果。例如：
There is no mother but loves her own children.(=There is no mother that/who does not love her own children.)沒有不愛自己孩子的母親。
(2)as可指人或物，常在限制性定語從句中充當主、賓、表語。
as引導的限制性定語從句的先行詞前面常有as /such /the same等修飾詞。例如：
He is as brave a man as ever berthed.他是世界上最勇敢的人。
as作關系代詞，還可用來引導非限制性定語從句，as引導的定語從句可放在句首、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前通常有表示比較的詞。例如：
Fewer guests than we had expected came to the party.來參加晚會的客人比我們預料的還要少。
