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, much, few, any, little等作先行词时,只用that,不用which.
c) 先行词有the only, the very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..
e) 先行词既有人,又有物时.
举例：
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.