定语从句在英语学习中非常重要，在各种考试中，定语从句的内容也占有相当大的比例，尤其是在阅读理解题里最为常见。如果对定语从句掌握不透，对内容的理解就会出现偏差，从而影响答题效果。
定语从句，顾名思义，在句子中充当定语，可修饰名词或代词。被定语从句修饰的名词或代词叫做先行词，而定语从句通常出现在先行词之后，由引导词引出。请大家记住这两个概念，方便后面学习。
例：This is the book which I bought yesterday.
上句中，book为先行词，which为引导词。
引导词一般为关系代词或关系副词。关系代词有：who, whom, whose, that, which等。关系副词有：when, where, why等。
易错点1：关系代词在定语从句中作主语时，从句谓语动词的人称和数要和先行词保持一致。这个规则一定要记住，因为也是经常出现的考点，且大家容易出现错误的地方。
例：This is the student who wants to see you.
上一句中，先行词是student，是单数第三人称，而引导词who在定语从句中作主语，这时候定语从句的谓语动词的人称和数要与student（先行词）一致，所以应该用单三形式。
例：There are some students who want to see you.
上一句中，先行词是students，是复数形式，而引导词who在定语从句中作主语，这时候定语从句的谓语动词的人称和数要与students（先行词）一致，所以应该用复数形式。
易错点2：引导词that不可以置于介词后引导宾语从句。
例：This is the house in which I lived two years ago.
上一句中，介词in后面的引导词不可以用that,只能用which.
易错点3：引导词用关系代词或关系副词分不清。这是一个非常容易犯的错误。
例：This is the house where I lived two years ago.
上一句中的先行词是house，引导词用了关系副词。没有问题。再看一看下面的例子。
例：This is the house which I bought last week.
上一句中的先行词是house，而引导词却用了关系代词。为什么呢？下面介绍判断的方法供大家参考。
如果定语从句中的谓语动词是及物动词，且无宾语，则应该用关系代词；如果谓语动词是不及物动词，则要用关系副词。上面两个例子中，live是不及物动词，引导词用了关系副词；bought是及物动词，且定语从句中没有宾语，所以要用关系代词。
（错）This is the school where I visited last year.
（对）This is the school which I visited last year.
（错）I will never forget the days when I spent with you.
（对）I'll never forget the days which I spent with you.
很多学生习惯于把表地点或时间的名词与关系副词where, when联系在一起，看到就盲目地使用，而不对从句进行具体分析，导致错误。
易错点4：引导非限定性定语从句时，不可以用that。这一点也是同学掌握不好的，常常弄错。
（错）The house, that we bought last week, is very expensive.
（对）The house, which we bought last week, is very expensive.
易错点5：不定代词anything，nothing，the one，all，much，few，any，little等作先行词时，只用that，不用which。
（错）All which is needed is time.
（对）All that is needed is time.
易错点6：先行词被the only，the very修饰时，只用that。
（错）This is the only book which I have.
（对）This is the only book that I have.
易错点7：先行词被序数词或形容词最高级所修饰时，只用that。
（错）This is the most interesting book which I have ever read.
（对）This is the most interesting book that I have ever read.
易错点8：先行词既有人，又有物时，只用that。
（错）He talked of things and persons which he remembered.（对）He talked of things and persons that he remembered.