1．定语从句：修饰某一名词或代词的从句叫定语从句。定语从句一般紧跟在它所修饰的先行词之后。
2．先行词：被定语从句修饰的词叫先行词。
3．关系词：引导定语从句的词叫关系词。
关系词有关系代词和关系副词。关系代词有that, which, who, whom, whose, as等；关系副词有when, where, why等。
关系词通常有下列三个作用：A、引导定语从句；B、代替先行词；C、在定语从句中担当一个成分。例如：
The man who is shaking hands with my father is a policeman. 该句中，who is shaking hands with my father 是定语从句，修饰先行词the man， “who”是引导定语从句的关系词，代替先行词the man，在定语从句中作主语。
as 和which 引导非限制性定语从句时， 其用法有相同之处，也有不同之处。具体情况是：
1. as 和which都可以在定语从句中做主语或宾语，代表前面整个句子。如：
He married her, as / which was natural. 他跟她结婚了，这是很自然的事。
He is honest, as / which we can see. 他很诚实， 这一点我们看得出来。
2. as 引导的非限制性定语从句可以放在主句之前、主句之后，甚至还可以分割主句。 which 引导的非限制性定语从句只可放在主句之后。另外，as 常常有“正如、正像”的含义。如：
As is known to all, China is a developing country. 众所周知，中国是发展中国家。
He is from the south, as we can know from his accent. 他是南方人，这一点我们从他的口音可以知道。
John, as you know, is a famous writer. 正如你所知， 约翰是个著名作家。
Zhang Hua has been to Paris more than ten times, which I don’t believe. 张华已去过巴黎十多次了，这一点我不相信。
注意：当主句和从句之间存在着逻辑上的因果关系时，关系词往往只用which。如：
Tom was late for school again and again, which made his teacher very angry. 汤姆老是迟到，这使得老师很恼火。
These tables are made of metal, which made them very heavy. 这些桌子是金属的，这使得这些桌子很重。
1. 当先行词受such, the same 修饰时，关系词常用as。 如：
I’ve never heard such stories as he tells. 我从未听过象他讲的这样的故事。
He is not such a fool as he looks. 他可不象他看上去的那样傻。
This is the same dictionary as I lost last week. 这部词典跟我上星期丢失的一样。
注意：当先行词受the same 修饰时，偶尔也用 that引导定语从句，但与as引导的定语从句意思有区别。如：
She wore the same dress that she wore at Mary’s wedding. 她穿着她在玛丽婚礼上穿过的同一条连衣裙。
She wore the same dress as her younger sister wore. 她穿着跟她妹妹所穿的一样的连衣裙。
最后，what 不可以引导定语从句的。
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