限制性定语从句与非限制性定语从句是英语语法知识中的一个难点，也是各类外语考试的必考内容。现将初中阶段的相关内容稍作总结，希望对大家有用。
1 何为限制性定语从句？
当我们需要用句子描写人或物时，可以使用定语从句。定语从句用来修饰一个名词或代词。其中被修饰的名词或代词称为先行词。定语从句通常出现在先行词之后，这样的句子就叫限制性定语从句。比如：
A detective is someone who looks for clues for important things.侦探是为重要事物寻找线索之人。
We are now checking the scene for more clues which will help solve the case.我们在检查现场，以查找更多有助于侦破此案的线索。
2 何为非限制性定语从句？
当先行词是专有名词或物主代词和指示代词所修饰时，其后的定语从句通常是非限制性的，如：
This novel，which I have read three times，is very touching.这本小说很动人，我都读三遍啦。
Charles Smith,who was my first teacher,retired last year.查理·史密斯先生去年退休了，他是我的启蒙老师。
说白了就是，限制性定语从句和非限制性定语从句的区别常常在于有没有多一个逗号。
3 如何选用引导词？
本文总结了三种情况来分别介绍。第一种情况，根据先行词选用。
A.当先行词是人，并且在定语从句中作主语是，用who。例子如下：
This is the man who will give us a talk.这就是将要给我们做报告的那个人。
B.当先行词是人，却在定语从句中作宾语时，引导词就要用whom了。如：
She was the woman whom you saw just now.她就是刚才你见到的那个人。
C.当先行词是人并表示所属关系，而且在定语从句中作定语时，引导词用whose.如：
Her mother wouldn't let her marry anyone whose family was poor.她妈不让她嫁给穷人。
D.当先行词是物，并在定语从句中作表语或既指人又指物是，引导词用that.如：
It's a book that will help you a lot.这是一本对你很有帮助的书。
We are deeply impressed by the teachers and schools that we had visited .我们对访问过的老师和学校印象深刻。
E.当先行词在定语从句中作状语，表示时间时用when引导，表示地点用where.如：
I never forget the days we stayed on the farm.我永远不会忘记我们在农场的日子。
This is the place where we lived.这是我们曾经居住过的地方。
第二种情况，根据先行词的修饰成分来选引导词。下面全是用that的例子。
A.当先行词被形容词最高级修饰时。如：
This is the most important task that should be finished soon.这是需尽快完成的最重要的任务。
B.当先行词被all,much,something,everything,any,few,little,no,some，last，next，the very，the only，the just等修饰时，如：
There's nothing that can be said about it.关于这事无话可说。
There's little work that is fit for you .几乎没有适合你的工作。
He sits the last row that is next to the window.他坐在靠窗的最后一排。
The only thing that I need now is time.我现在需要的是时间。
This is the very book that you want to borrow.这就是你想借的书。
C.主句是以who,which引导的疑问句时。如：Who is the woman that was talking with her mother?和她妈妈聊天的那位女士是谁？
第三种，根据实际情况选引导词。遇到以下情况，只能用which。
A.引导词后面有介词。如：
This is the room in which he works.=This is the room where he works.这是他的工作间。
B.先行词本身是that.如:The watch is that which tells the time.表是用来报时的。
C.非限制性定语从句中先行词是物时。如：He has a computer,which was bought by his mother.他有台电脑，是妈妈给买的。
Last year my father told me a story,which I remember clearly today.去年爸爸给我讲的那个故事，至今我记忆犹新。
D.引导词在被动句中作主语时。如：I like the toys which are colored.我喜欢那些涂色的玩具。
以上就是这篇小文对定语从句的核心语法知识进行的简单梳理，希望大家有所收获。
