1、 定语从句在句中修饰某个名词或代词,被定语从句修饰的词叫先行词.定语从句需用下列关联词：
起连词作用,本身做从句的主语、宾语、表语或定语 起连词作用,本身在从句中作时间、地点或原因状语
who,whom,that,whose,which,as when,where,why
2、 掌握以上关系代词、关系副词的含义及使用时,要特别注意以下几点：
1） 指人时宜用who 的情况：
a.当先行词是 one,ones,anyone 或 those 时,关系代词用 who.
Anyone who goes there will be punished.
b.在there be 开头的句子中.
There’s a beautiful girl who came to see you this morning.
c.先行词后有一个较长的定语.
I met a foreigner in the city last week who could speak Chinese well.
d.在非限定性定语从句中.
She has a brother,who worked at that factory ten years ago.
2) 指物时宜用 that 的情况：
a.当先行词为 all,much,little,few,none,something,anything,everything,nothing 等不定代词时.All that can be done has been done.
b.当先行词既指人又指物时.
He spoke of the men and the thing (that) he had seen abroad.
c.当先行词被序数词或形容词的最高级修饰时.
This is one of the best films that I have ever seen.
d.当先行词被 the very,the only,the last,any,every 等修饰时.
This is one of the very book that I am looking for.
e.当先行词是疑问词 who,what,which 时.
Who that has such a home doesn’t love it?
f.关系代词在从句中作表语.
Mary is no longer the girl that she used to be.
3) 只能用 which 不能用 that 的情况.
a.引导非限定性定语从句修饰某物或整个句子.
Bruce went towards the fire,which was still smoking.
Tom came back late,which made his parents very angry.
b.关系代词作介词的宾语.This is the house in which Lu Xun once lived.
4) 关系代词 as 和which都能引导非限定性定语从句代表整个句子的内容,as
引导的从句可位于句首或句末,which引导的定语从句则不能位于句首.
Which you know,he is a good man.(×)
As you know,he is a good man.(√)
6)关系副词when,where,why其含义相当于on which,in which for which等,可以互换：
The day when /on which I met him first was May 1st.
I don’t know the reason why /for which he didn’t come.
7)whose指物时,可以与of which等结构互换,但应注意与冠词的位置关系：
This is the book the cover of which / of which the cover / whose cover is blue.
8)有时可用代替关系副词.在口语中常省略.
This is the reason (why / for which / that) he came late.
9)先行词是专有名词、整个句子或世界上独一无二的物质名词时,一般用非限定性定语从句修饰.The sun,which gives us light and heat,is very big.
10)在先行词和定语从句之间有无逗号有时会引起名义的变化：
He said nothing that made her angry.他没说使她生气的话.
He said nothing,which made her angry.他一言不发,这使她很生气.
3、 定语从句与强调结构
It is the place where they lived before.
It is in the place that they lived before.
第一个句子为定语从句,where指代the place,在定语从句中作状语,第二个句子为强调结构,强调in the place,that没有意义,把放回后面句子,句子意思完整.
Where is it that he found the lost watch?(强调句型,强调疑问副词where.)
Where is the watch he found yesterday?(定语从句,that指代the watch.)
4、 定语从句中的先行词
Is this book the one that you bought yesterday?
Is this the book that you bought yesterday?
第一个句子中,this book是主句的主语,the one是先行词.在第二个句子中this是主句的主语,the book是先行词.一定要避免出现：Is this book that you bought yesterday?
定语从句相当于形容词,它对先行词起修饰、描述或限制作用,而同位语从句则相当于名词,对其前面的词给予说明或作进一步解释,即说明该词所表示的具体内容.例如：
The news that we heard is not true.(定语从句)
The news that he won the prize is not true.（同位语从句）
另:在“have no idea ＋从句”结构中,其从句都作idea的同位语.例如：
I have no idea when she will be back.
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