1.定語從句的特征
定語從句分為限定性定語從句和非限定性定語從句。限定性定語從句與主句關(guān)系緊密，為句中不可缺少的成分，如去掉，則主句意思不完整或不明確。非限定性定語從句與主句關(guān)系松散，如去掉，主句內(nèi)容仍完整。在書面語中，非限定性定語從句一般用逗號與主句隔開。
引導(dǎo)定語從句的關(guān)聯(lián)詞包括
(1)關(guān)系代詞：that，which，whose，who, whom，as。
(2)關(guān)系副詞：when，where，why。
關(guān)系代詞和關(guān)系副詞在定語從句中都充當(dāng)一定的句子成分：主語(who，which，that)；賓語(that，which，whom)；定語(whose，of which)；狀語(when，where，why)
2.定語從句中關(guān)系詞的運用
(1)關(guān)系代詞
關(guān)系代詞的選擇一般需從三方面來考慮：①先行詞是指人還是指物；②關(guān)系代詞在從句中作主語、賓語還是定語；③該從句是限定性定語從句還是非限定性定語從句。
關(guān)系代詞作賓語時，一般可省略。如：
I know that he is a man who/that means what he says. (先行詞指人，關(guān)系代詞在限定性定語從句中作主語)
The man whom/that she met told her a lie. (先行詞指人，關(guān)系代詞在限定性定語從句中作賓語)
The watch which/that was lost has been found. (先行詞指物，關(guān)系代詞在限定性定語從句中作主語)
Here is the material which/that you need. (先行詞指物，關(guān)系代詞在限定性定語從句中作賓語)
He is the only one whose advice she may accept. (先行詞指人，關(guān)系代詞在限定性定語從句中作定語)
(2)關(guān)系副詞。
關(guān)系副詞的選擇主要看先行詞。如先行詞為表示時間的名詞，如time，day等，則用when；如先行詞為表示地點的名詞，如place，house，area等，則用where；如先行詞為reason，則用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)關(guān)系代詞that和 which的區(qū)別。①存在以下情況之一時，只能用that來引導(dǎo)定語從句：
第一，先行詞是all，everything，something，anything，little，nothing等不定代詞時；
第二，先行詞前有next，first，last，only，few，much，some，any，very，no等詞修飾時；
第三，先行詞前有形容詞最高級修飾時。例如：
I've explained everything (that)I can to you. This is the most beautiful city (that)I've ever been to.
②非限定性定語從句一般不用that引導(dǎo)，且關(guān)系詞不可省略。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引導(dǎo)定語從句的區(qū)別。在非限定性定語從句中，先行詞是整個句子時，一般用which或as來引導(dǎo)。which在從句中可充當(dāng)主語、賓語等，as在從句中一般只充當(dāng)主語。which與as引導(dǎo)此類定語從句的區(qū)別在于：which只能置于句中或句末，而as的位置比較靈活，既可置于句中、句末，又可置于句首。此外，as引導(dǎo)定語從句時還常和such，so，as或same連用，一般沒有明顯的先行詞，as一般可以譯為“正如，就像…那樣”。
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
答案為［A］。As指代的是后面整句話的內(nèi)容，引導(dǎo)非限定性定語從句，其他選項只能引導(dǎo)主語從句。本句意為：正如我們從比較這些數(shù)字所看到的結(jié)果一樣，這項規(guī)則需要病人在治療時積極參與。
(5)關(guān)系代詞在定語從句中有時也作介詞的賓語。如果介詞被置于關(guān)系代詞前，一般只用介詞+which或介詞+whom，而不能用介詞+that來引導(dǎo)定語從句。如果介詞被置于從句句末，則可用that代替which或whom，且that這時可省去。
This is the coat on which she spent 500 dollars.
One of my classmates whom/that you are familiar with will come today.
Living in the central Australian desert has its problems, _____ obtaining water is not the least.(1994.3)
［A］ of which［B］ for what［C］ as［D］ whose
答案為［A］。題目中which的先行詞是problems，在which引導(dǎo)的非限定性定語從句中作介詞of的賓語，這時關(guān)系代詞要置于介詞后面，不能省略。介詞of表示所屬關(guān)系，of which指的是“在這些問題中”。［B］ 中的for表示原因，與題意不符；［C］ 和［D］ 也能引導(dǎo)定語從句，但都與句子的意思不符，所以只能選［A］。本句句意為：在澳大利亞中部，沙漠生活面臨許多問題，其中汲水是一個很嚴(yán)峻的問題。
2.定語從句與同位語從句的區(qū)別
同位語從句大多由連接詞that引導(dǎo)，常跟在下列名詞后面，如fact，idea，opinion，news，hope，belief等。同位語從句用來解釋說明這些名詞的具體含義和內(nèi)容。定語從句對所修飾的名詞加以修飾限定，即說明其為此人(事物)而非彼人(事物)。如：
We are familiar with the idea that all matter consists of atoms.(同位語從句)
The news that we are invited to the party is very encouraging.(同位語從句)
This is the best article that has been written on the subject. (定語從句)
4.定語從句與what引導(dǎo)的名詞性從句的區(qū)別
what只能引導(dǎo)名詞性從句，在從句中充當(dāng)主語、賓語或表語，但總是包含著定語從句的含義，很容易與定語從句混淆。定語從句的引導(dǎo)詞前面都會有先行詞，而what前面則沒有任何先行詞，此時what=先行詞+that。試比較：
He always means what he says.
That's the only thing that he says.
That's what I want to say.
That's all (that) I want to say.
This is what I can give you."
This is all the money that I can give you.
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