(1)不用that的情况
(a) 在引导非限定性定语从句时。
(错)The tree, that is four hundred years old, Is very famous here.
(b) 介词后不能用。
We depend on the land from which we get our food.
We depend on the land that/which we get our food from.
(2) 只能用that作为定语从句的关系代词的情况
(a) 在there be 句型中，只用that，不用which。
(b) 在不定代词，如：anything, nothing, the one, all等作先行词时，只用that，不用which。
(c) 先行词有the only, the very,the same,the last,just修饰时，只用that。
(d) 先行词为序数词、数词、形容词最高级时，只用that,不能用which。.
(e) 先行词既有人，又有物时。
(f) 先行词指物，在主句中作表语时．
(g) 为了避免重复．
(h)先行词是the way或the reason时，that可作关系副词，也可省略
(i) 主句的主语是疑问词who /which时
举例：
Is this the book that you borrowed in the library?
这是你在图书馆借的那本书吗？
Who that break the window should be punished．
谁打碎了窗户都要受到惩罚．
All that is needed is a supply of oil.
所需的只是供油问题。
Finally, the thief handed everything that he had stolen to the police.
那贼最终把偷的全部东西交给了警察。
that必用
1先行词是最高级或被最高级修饰
2是序数词或被序数词修饰
3不定代词
4先行词既指人又指物
5被only；the every，no，one of等修饰
6主句有which；who；whom
7there be 句型中
8如果有两个从句，其中一个关系代词已用了which,另一个关系代词宜用that
9先行词在主句中作表语，或者关系代词本身作从句的表语是宜用that
that在作宾语时可省
10.不是任何时刻关系词作主语宾语是能用that