只能用“that”的情况
1.先行词为不定代词（all,nothing,the one,
much等）或被不定代词修饰时
You
can take any seat that is free.Tell
us all that you know.
2.先行词是序数词或被序数词修饰是,如：
That is the first composition that I’ve
written in English.
3.先行词是形容词最高级或被形容词最高级修饰时,如：
This
is the best that has been used against pollution.
4.当人和物同为先行词时,如：
Everyone wants to see the spaceship and the
spaceman that made the flight around the earth.
5.关系代词在从句中做表语时,如：
He is no longer the man (that) he used to be.
6.先行词被only,the same,the very等修饰时,如：
That
is the very pen (that) I am looking for.
7.当先行词是which时,如：
Which of the two cows that you keep produces
more milk?
8.先行词为the way/ the time/ the
moment/ the first time/ the last time等时间名词时,如：
I
don’t like the way (that) he talks.
9.定语从句中套定语从句,其中一个关系代词已用which时,如：
He built a factory which produced things that
had never been seen before.
（4）只能用“which”的情况
1.关系代词前有介词或介词短语时,如：
Here is the book about which I told you yesterday.
2.非限定性定语从句中,如：
The
weather turned out to be very good,which was more than we could expect.
1.先行词后有插入语时,如：
Here is the English
grammar book which,as I’ve told you,will help improve your English.