定语从句就是在考句子成分，而关系代词与关系副词的区分就是为了划定句子成分。这也就是为什么要熟背出所有的关系代词和关系副词的道理。关系代词对人有who,whom,对物有which,都可以的是that,说谁的是whose,还有一个特殊的as.关系副词只有三个，时间地点和原因，when,where和why.
正由于关系代词和关系副词在句子中所做成分截然不容，其中关系代词作的是主语，宾语（表语也算，和宾语位置相同），还有whose作的是定语，而关系副词永远做的是状语。所以大多数情况下（whose表示物主一会儿另说）当横线后定语从句不缺少主语或者宾语的情况下，就必然不会填关系代词中的任何一个，这会儿往往就已经筛掉了2个选项了。剩下三个关系副词具体是谁，就看先行词就行了。
到这里，还要多说一点，关系副词是可以由“介词+which”替换的，比如in the morning的定语从句改写就既可以改写为when也可以改写为in which,但是关系副词why的替换是固定的，由于先行词基本就是reason,所以why只能替换为for which。
下边来说一下特殊情况，只能用that的几种情况，挨个背一下吧。不定代词，all,only,very,序数词，最高级，又有人又有物。
然后是as这个特殊的词，引导非限制性定语从句的时候，as可以放在句首，而which不可以。之后就是such as 和such that的问题了。such as本身当“比如”讲，但是such 和as之间也是可以放词的，就成了such A as B,这时意思就成了“像B这样的A”,就成了定语了，也就是as是关系代词了，这代表什么信息呢？很重要，由于关系代词as是必须要在定语从句中作主语或宾语的，因此as后边引导的句子必然缺少这两个成分的其中一个，才能填as.说这么多是为什么呢？因为such有两个词组，分别是such … as… 和such… that…，考试最常考的也是它们俩。区别很简单，such that表示“如此以至于”,引导的是结果状语从句，而such… as… 引导的是定语从句。
下边我用一个实际的句子来诠释。
He is such a good boy ______ everybody likes.
He is such a good boy ______ everybody likes him.
两个句子唯一的区别就在于第2句多了个him,因此第2句主谓宾全齐了，不需要再填关系代词了，所以就只能用that引导，表示结果状语从句。而第1句恰好相反，缺少了宾语，必须用as来作宾语。这个问题到此说清。
接下来说whose作定语的问题，表示所属关系不见得是人的，“物的”也是可以的，比如房子的窗户在定语从句中有三个说法，the house whose window = the window of which = of which the window.这个就叫做所属关系，因此所属关系除了whose以外就有两个，对人是of whom,对物是of which。到这儿，就又出现了另一个常见考点，就是of which/whom 和of them的区别是什么呢？也很简单，of which/whom是定语从句，也就是不能单独成句的句子，而of them是可以单独成句的句子，不是定语从句，举例如下：He had many friends, many of whom are foreigners. 是定语从句。He had many friends, and many of them are foreigners. 就是并列句了，由and引导的句子叫并列句。
最后是关系代词的省略问题，关系代词作宾语或表语是可以省略的，作主语是不能省略的，因为主语后直接是动词，省略的话就出现先行词直接接从句动词的语法错误。举例：Is there anything that you want to say? 中的that就可以省略。而He is the headmaster of this school who also teaches Chemistry. 中的who就当然不能省略。
好了，说了这么多，基本上把定语从句的脉络理清，接下来用几个常见考题来巩固一下。
（高考真题） 31. The prize will go to the writer ________ story shows the most imagination.
A. that B. whichC. whose D. what
解析：【C】whose作定语，表示“谁的”.
（高考真题）7. Ted came for the weekend wearing only some shorts and a T-shirt, ____ is a stupid thing to do in such weather.
A. this B. that C. what D. which
解析：【D】非限制性定语从句，先行词为前边整句话，后边直接接的是动词is,所以用which作主语。
（高考真题）26. Mary was much kinder to Jack than she was to the others, _________, of course, make all the others upset.
A. whoB. whichC. whatD. that
解析：【B】同上一题。
（高考真题） 39. You'll find taxis waiting at the bus station ____ you can hire to reach your host family.
A. whichB. whereC. whenD. as
解析：【A】hire是“雇佣”的意思，这里就是租车了，所以You can hire 是主语谓语，to reach your host family 是目的状语，这个句子缺少宾语，即租的“车”,所以缺宾语用代词，which.
（高考真题）32.The old town has narrow streets and small houses _____are built close to each other.
A. they B. where C. what D. that
解析：【D】横线后直接接动词are,要用关系代词作主语，只有that了。
（高考真题）34. She showed the visitors around the museum, the construction _____had taken more than three years.
A. for whichB. with whichC. of whichD. to which
解析：【C】这就是所属关系，the construction of the museum = whose construction = of which the construction
（高考真题）24. Between the two parts of the concert is an interval, _______ the audience can buy ice-cream.
A. when B. where C. that D. which
解析：【A】 the audience 为主语，can buy为谓语，ice-cream为宾语，所以从句完整，不能用关系代词。所以要用关系副词，先行词是interval,表示“间隔”,是时间，所以用when.
（高考真题）25.Julie was good at German, French and Russian, all of _________ she spoke fluently.
A. who B. whom C. which D. that
解析：【C】同样是所属关系，对物所属，所以用of which.
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