用一个句子作定语，这个句子就叫定语从句。定语从句通常紧靠在它所修饰的名词或代词后面。定语从句相当于一个形容词，用来修饰名词、代词或句子。

17-5 习惯上要用that引导定语从句的5种情况

that引导定语从句，既可以指人，相当于who，也可以指物，相当于which，但下列5种情况通常用that而不用who或which。

1.先行词为指物的不定代词或被不定代词修饰时

当先行词是指物的all，little，few，much，any，anything，everything，nothing，none，the one时，或先行词被all，little，few，much，any，every，no等修饰时用that不用which。如：

All that can be done has been done.能做的都已经做了。

He will tell you everything that he heard about it .他将告诉你他所听到的关于这件事的一切。

There is little work that is fit for you .没什么工作适合你做。

I have eaten all the food that is left .我把剩下的所有食物都吃了。

2.先行词为序数词、最高级或被序数词、最高级修饰时

当先行词是序数词或最高级形容词时，或者先行词被序数词（包括last，next）、最高级形容词等修饰时用that不用which。如：

This is the first film that I have seen since I came here .这是我到这里后所看的第一场电影。

This is the best method that has been used against pollution .这是用来防止污染的最好办法。

3.先行词包括人和物时

当先行词包括人和物两项时用that不用who或which。如：

They often talk about the people and the things that they are interested in .他们常常谈论那些他们感兴趣的人和事。

4.主句为who或which开头的疑问句时

当主句是以who或which开头的疑问句时，为了避免重复，多用that引导。如：

Which is the house that caught fire last night ？昨晚失火的是哪座房子？

Who is the lady that is playing the piano ？在弹钢琴的那位小姐是谁？

5.先行词被the only，the very，the same等修饰时

当先行词被the only，the very，the same，the right等修饰时，用that不用which。如：

He is the only boy that is invited .他是唯一的一个被邀请的男孩。

This is the very book that I am looking for .这正是我一直在寻找的那本书。