首先，只有先行词是物，才有以下用法
当先行词为不定代词，如anything, everything, nothing, none, one, all, little, few, much 等，或先行词被all, little, few, much, any, only, the every, every, no等修饰时，要用that指代先行词
All that is needed is oil.
Finally, the thief handed everyting that he had stolen to the police.
This is the only problem that remains to be settled.
2. 先行词是序数词/最高级形容词时，或先行词被序数词，最高级形容词修饰时，要用that
This is the first/best film that I have seen since I came here.
3. 先行词同时包括人和物时，要用that
They often talk about the people and the things that they like most.
4. 避免重复
① 主句的主语是疑问词who或which
Which is the bike that you have lost?
Who is the boy that won the gold medal?
② 两个定语从句，其中一个用which另一个用that
They secretly built up a small factory, which produced things that could cause much pollution.
5. 先行词在主句中作表语,关系词在从句中也作表语
Shanghai is no longer the city that it used to be.
6. 主句是there be 句型且关系词在从句中作主语
There is a seat that is still available.
7.限定性定语从句中，万一脑抽无法判断的时候，选that居多