(一)定语从句
1.定语从句的特征
定语从句分为限定性定语从句和非限定性定语从句。限定性定语从句与主句关系紧密，为句中不可缺少的成分，如去掉，则主句意思不完整或不明确。非限定性定语从句与主句关系松散，如去掉，主句内容仍完整。在书面语中，非限定性定语从句一般用逗号与主句隔开。
引导定语从句的关联词包括：
(1)关系代词：that，which，whose，who, whom，as。
(2)关系副词：when，where，why。
关系代词和关系副词在定语从句中都充当一定的句子成分：主语(who，which，that)；宾语(that，which，whom)；定语(whose，of which)；状语(when，where，why)。
2.定语从句中关系词的运用
(1)关系代词。
关系代词的选择一般需从三方面来考虑：①先行词是指人还是指物；②关系代词在从句中作主语、宾语还是定语；③该从句是限定性定语从句还是非限定性定语从句。
关系代词作宾语时，一般可省略。如：
I know that he is a man who/that means what he says. (先行词指人，关系代词在限定性定语从句中作主语)
The man whom/that she met told her a lie. (先行词指人，关系代词在限定性定语从句中作宾语)
The watch which/that was lost has been found. (先行词指物，关系代词在限定性定语从句中作主语)
Here is the material which/that you need. (先行词指物，关系代词在限定性定语从句中作宾语)
He is the only one whose advice she may accept. (先行词指人，关系代词在限定性定语从句中作定语)
(2)关系副词。
关系副词的选择主要看先行词。如先行词为表示时间的名词，如time，day等，则用when；如先行词为表示地点的名词，如place，house，area等，则用where；如先行词为reason，则用why。另外，where = in which，why = for which。例如：
I'll never forget the village where/in which I spent my childhood.
I don't know the reason why/for which he did that.
I'll never forget the day when I first entered the college.
(3)关系代词that和 which的区别。①存在以下情况之一时，只能用that来引导定语从句：
第一，先行词是all，everything，something，anything，little，nothing等不定代词时；
第二，先行词前有next，first，last，only，few，much，some，any，very，no等词修饰时；
第三，先行词前有形容词最高级修饰时。例如：
I've explained everything (that)I can to you. This is the most beautiful city (that)I've ever been to.
②非限定性定语从句一般不用that引导，且关系词不可省略。例如：
His lecture, which bored everyone, ended at last.
The teacher's daughter, whom I met last month, sent me an e-mail.
(4)which和as引导定语从句的区别。在非限定性定语从句中，先行词是整个句子时，一般用which或as来引导。which在从句中可充当主语、宾语等，as在从句中一般只充当主语。which与as引导此类定语从句的区别在于：which只能置于句中或句末，而as的位置比较灵活，既可置于句中、句末，又可置于句首。此外，as引导定语从句时还常和such，so，as或same连用，一般没有明显的先行词，as一般可以译为“正如，就像…那样”。
例如：Water consists of hydrogen and oxygen, which/as is known to all.
As everybody knows, he is a good boy.
Such things as you have described are most important to me.
He is that kinds of man, such as people always meet.
I have the same dictionary as you have.
_____can be seen from the comparison of these figures, the principle involves the active participation of the patient in the modification of his condition.(1999.5)
［A］ As［B］ What［C］ That［D］ It
答案为［A］。As指代的是后面整句话的内容，引导非限定性定语从句，其他选项只能引导主语从句。本句意为：正如我们从比较这些数字所看到的结果一样，这项规则需要病人在治疗时积极参与。
