1）不需要that的状况
a) 在引导非限定性定语从句时。
The tree, that is four hundred years old, is very famous here.
b) 介词后不可以用。
We depend on the land from which we get our food.
We depend on the land that/which we get our food from.
2) 只可以用that作为定语从句的关系代词的状况
a)在there be 句型中，只用that，不需要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，不需要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 supply1 of oil2.
所需的只不过供油问题。
Finally, the thief handed everything that he had stolen to the police.
那贼最后把偷的全部东西交给了警察。