请用最简洁的语言告诉我that和which的区别,还有为什么看到有些句子介词后也可加that,

介词后面加THAT是“那个”的意思.比如：Your book is on that table.
that,which很好区别.说简单点,定语从句当中,有逗号隔开的,逗号后面用Which,因为是非限定性定语从句.
没有逗号,THAT 和 WHICH一般可以互换.但是有几种情况只能用THAT,这是考点.
1.当先行词为anything,everything,nothing等不定代词时,只能使用“that”.例：Is there anything that I can do for you?我能给你做点什么吗?
2.当先行词为all,any,much,many词时,只能使用“that”.例：Tom told her mother all that had happened.汤姆把事情的全部情况告诉了他的妈妈.
3.当先行词是形容词最高级或被形容词最高级修饰时,只能使用“that”.例：This is the most wonderful time that I have ever had.这是我度过的最美好的时光.
4.当先行词是序数词修饰时,只能使用“that”.例：He was the first person that passed the exam.他是第一个通过考试的人.
5.先行词被the only,the very,the same,the last修饰时,只能使用“that”.例：1.This will be the last chance that he can get.这将会是他得到的唯一机会.2.He is the only person that can help you out.他是唯一能帮你的人.
6.先行词里同时含有人或物时,只能使用“that”.例：They talked of things and persons that they remembered in the school.他们谈论他们记得的有关学校的事和人.
7.当主句是以who,which 开头的特殊疑问句或先行词是who时,只能使用“that”.例：Which is the book that he bought from the library yesterday?哪本是你们昨天从图书馆借的书?
这是最简单的说法了.