定语从句也称为关系从句或形容词性从句，由关系代(who, whom, whose, which, that, as )或关系副词(when, where, why)等引导。关系词除了起引导作用外，还在从句中指代前面的先行词，并在从句中担任一定的成分。下面是yjbys小编为大家带来的关于定语从句的语法知识，欢迎阅读。
1、限定性定语从句和非限定性定语从句
定语从句就其与先行词的关系而言，可分为限定性定语从句和非限定性定语从句。限定性定语从句和先行词之间没有逗号，表示对先行词起修饰限制作用;而非限定性定语从句和先行词之间有逗号分开，是对先行词进行补充说明，相当于并列句。
如：He is the man who has a strong personality. 他是个个性很强的人。(限定性定语从句)
My brother, who works abroad, is coming next week. 我哥哥在国外工作，他下周将回来。(非限定性定语从句)
注：
1) 非限定性定语从句不能用that来引导，一般用which，as或who(指人)。用which或as引导时，既可以修饰主句的部分内容，也可修饰主句的全部内容。如：
He spoke confidently, which impressed me most. (which指代整个主句，在从句中作主语)
2) as引导的非限定性定语从句既可以放在主句之前，也可以放在主句之后，但which或who引导的非限定性定语从句不能放在主句之前。如：As we all know, the moon is a satellite of the large area is covered with thick snow, which affects people‘s life greatly.
3) 在限定性定语从句中，关系代词作宾语时可以省略;但在 非限定性定语从句中，关系代词作宾语时不能省略。如：
The film (which) I saw last night is about a young teacher. (which可以省去)
She introduced me to her husband, whom I hadn’t met before. (whom不能省去)
2、只用关系代词that，不用 which，who或whom的几种情况
1)当先行词是all, everything, nothing, something, anything, little, much, none, few等时。如：All that you want are here.
2) 当先行词被all, no, some, any, every, a few, a little, much, only, very等修饰时。如：There is no person that doesn’t make mistakes.
3)当先行词被序数词或形容词最高级修饰时。如：This is the best film that I have ever seen.
4)当出现两个或两个以上的先行词，并同时兼指人和物时。如： We are talking about the people and countries that we have
visited.
5) 当主句是以which 或 who 开头的特殊疑问句时。如： Who is the person that is standing over there?
6) 当定语从句为there be 句型时，关系代词只用that，
但经常可以省略。如： I know the difference (that) there is between you.
7)当关系代词在从句中作表语时，常用that。如：He does not seem to be the man that he was
他似乎和过去不一样了。
3、 as 与其他词连用引导的定语从句
as 引导定语从句时，可指人也可指物，通常与the same，such，so等连用，形式为：the same… as，such… as，as …as，so… as。As在定语从句中可作主语、宾语、表语等。如：
She knew he felt just the same as she did. 她知道他的感觉与她自己的一样。(as代替事，作宾语)
I lend you such books as will interest you. 我借给你使你感兴趣的书。(as代替物，作主语)
注：the same… as 和the same… that的用法不同。as引导的定语从句表示其内容与主句所讲的事物“相似”，指同类事物;that引导的定语从句表示其内容与主句所讲的事物是“同一个”，指同一事物。如：
This is the same pen as I lost yesterday. 这和我昨天丢的笔一样。(类似的笔)
This is the same pen that I lost yesterday. 这支笔就是我昨天丢的`那支。(同一支笔)
4、“介词+关系代词”引导的定语从句
在“介词+which/whom”结构中，介词的选择取决于3种情况：
1)定语从句中谓语动词或表语的搭配。如： This is the college in which I am studying. He is the man about whom we are talking.
2)先行词与介词的习惯搭配。如： The speed at which the machine operates is shown on the meter.
仪表上显示出这台机器运转的速度。
3)当定语从句为最高级时只能用of which; 否则用其他介词。如： I have five dictionaries of which Longman Dictionary is the best. I have five dictionaries among which Longman Dictionary is published in UK.
注：不可拆分的动词短语介词不能提前，如put up with;有的短语可能有不同的介词搭配，此时需要根据上下文来确定。如：be familiar with/to，compare with/to等。
有时为表意清楚，还可以在关系副词where/ when前加介词from, to等。如：
China is the birthplace of kites, from
where kite flying spreads to Japan, Korea, Thailand and India.
中国是风筝的故乡，从这里，放风筝的运动传到了日本、韩国、泰国和印度。
注 定语从句和同位语从句的区别
定语从句在复合句中的功能相当于形容词，它对先行词起修饰、描述或限制作用。而同位语从句则属于名词性从句，对与它有同位关系得名词作进一步解释，使其内容具体化。例如：
She felt very sad because of the news that her husband was killed in an aircraft crash. (同位语从句)
The fact that she told me made me very angry. (定语从句)
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