which和what的用法？
1.当先行词为all, much, none, something, nothing, everything, the one, few, little, many, anything等不定代词时。例如：
We should do all that is useful to the people. 我们应该做一切有益于人民的事情。
2.当先行词被only, any, few, little, no, all, many, much等不定代词修饰时。例如：
I have finished reading all the books that I borrowed from the library. 我己经看完了所有从图书馆借来的书。
3.当先行词被序数词、形容词最高级以及the only, the very, the next, the last等修饰时。例如：
This is the first book that I bought in the shop . 这是我在这家书店买的第一本书。
4.当先行词是形容词最高级时。例如：
This is the best that can be done now.这是现在能做的最好的办法。
5.先行词既指人又指物时。例如：
We talked of things and persons that we remembered. 我们谈起了我们记住的人和事儿。
6.关系代词在限制性定语从句中作表语，并带有比较的含义时。例如：
Our school is no longer the school that it was ten years ago .我们学校已经不再是十年前的样子。
7.句子的前一部分已经出现了which,为避免重复，句子的后面部分通常不再用 which而用that来引导限制性定语从句。例如：
Which is the book that you bought yesterday? 哪一本书是你昨天买的？
8.当主句是以who , which 引起的特殊疑问句时，关系代词通常只用that。 例如：
Who is the man that you spoke to just now? 刚才和你说话的那个人是谁？
