一、定语从句的概念及关系词
1.定语从句的概念：
在复合句中，修饰某一名词或代词的从句叫定语从句。
2.先行词：
被定语从句修饰的名词或代词叫先行词。
3.关系词：
引导定语从句的连词叫关系词。如that、which、who、whom、whose、where、when等。
4.由that、which、who、whom、whose引导的定语从句。
that指人也指物
which指物
who指人
whom指人，做宾语
whose指人或物，做定语
二、关系词只能用that与which的情况
1.先行词是物时，一般情况既可用that也可用which.但下列情况下，只能用that不用which。
1).当先行词为指物的不定代词，如all、everything、something、anything、nothing、none、the one等时，只能用that。
There is nothing ___ I can do for you .
2).当先行词被the only、the very 、the last、all、no、little等词修饰时，只能用that。
This is the very book__ I’m looking for .
3).先行词被序数词修饰时，只能用that .
This is the first nextbook ______ I studied in the middle school .
4).先行词为最高级或被最高级修饰时，只能用that。
This is the most beautiful mountain _____ I have ever seen .
=I have ___ seen ___ a beautiful mountian .
5).先行词既有人也有物时，只能that 。
He told us many interesting things and persons _______ we had .
2.先行词是物时，一般情况既可用that也可用which.但下列情况下，只能用which不用that。
1).关系代词前有介词时，关系词只能用which 。
This is the building in ______ he lives .
2).先行词本身是that时，关系词只能用which 。
The clock is that _____ tells the time .
3).引导非限定性的定语从句（先行词是物且先行词与关系词用逗号隔开），关系词用which。
His book , ______ was lost last week , has been found now.
3.先行词是人时，一般情况下既可用who 也可用that 。但以下情况下，只能用who。
1).当先行词为those、one、ones、anyone且作主语时，一般用who 。
Those ______ are singing are all my classmates .
2).在there be 句型中，先行词指人时，只能用who。指物时用that。
There is a girl ___ expects to see you .
3).当先行词是I、you、he、they等时，只能用who 。
He _____ plays with fire gets burned .