1）不用that的情況
　　a) 在引導非限定性定語從句時。
　　　 (錯)　The tree, that is four hundred years old, is very famous here.
　　b) 介詞后不能用。
　　　　 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作為定語從句的關系代詞的情況
　　a)　在there be 句型中，只用that，不用which。
　　b)　在不定代詞，如：anything, nothing, the one, all, much, few, any, little等作先行詞時，只用that，不用which。
　　c)　先行詞有the only, the very修飾時，只用that。　
　　d)　先行詞為序數詞、數詞、形容詞最高級時，只用that。.
　　e)　先行詞既有人，又有物時。
舉例：
　 All that is needed is a supply of oil.
　 所需的只是供油問題。
　 Finally, the thief handed everything that he had stolen to the police.　
　 那賊最終把偷的全部東西交給了警察。