that和which在从句里都可以做宾语和主语，做宾语时可以省略。在定语从句中,有种说法叫"关宾省",意思是关系代词在限制性定语从句中作宾语可以省略,不管是that，which，还是whom。但是which能引导非限制性定语从句，that则不行。
1.先行词是人，用that/who/whom引导从句；不能用which：
如：He that/who fears death lives not.贪生怕死死的快。
2.先行词是物，which和that都可以用：
如：She has a small office which is used for private discussions.她有一间小办公室，用来私人会谈。
3.先行词是代词，用that/who不用which：
如：Is there anything else that you'd like?还再要点其他的吗?
4.先行词前有以下限定词，更多用that：
much, little, none, all, few, every, any, no；the very, the only, the same；形容词最高级或序数词
如：The best book that I read this year was 'The Da Vinci Code.'我今年读的最好的书是《达芬奇密码》
5.可以用介词加which，不能用介词加that：
如：Your metabolic rate is the speed at which your body transforms food into energy.新陈代谢的速度是你的身体将食物转化为能量的速度。
6.非限制性定语从句里，用which不用that：
如：The wisdom of nations lies in their proverbs, which are brief and pithy.民族之智慧蕴藏于其简短而有力的谚语中。
7.如果先行词在从句中做宾语，that/who/whom/which都可以省略：
如：The cell is the unit (that/which) all living organisms are composed of.细胞是构成所有生物的单位。
