问：什么情况下关系代词只能用that，而不用which?
　　
答： 不能用关系代词 Which 的情况有以下几种：

1.当先行词为all, little, much, few, everything, none 等不定代词时关系代词用that而不用which.      　　
　I will tell him all that you told me at the ball.
　There are few books that you can read in this book store.

2.当先行词既有人又有物时,关系代词用that, 不用which.      　　
　He asked about the factories and workers that he had visited.

3.当先行词有形容词最高级修饰时,关系代词用that, 而不用which.
　This is the best novel that I have ever read.     　　
　Guilin is one of the most beautiful cities that I have ever visited.

4.当先行词有序数词修饰时,关系代词用that, 不用which.    　　
　The first sight that was caught at the Great Wall has made a lasting impression on him.

5.当先行词被the only, the very, the same, the right等修饰时,关系代词用that,不用which.    　　　　  That is the very thing that we can do.    　　
　It is the only book that he bought himself.

6.当先行词被all, every, any, much, little, few, no等修饰时,关系代词用that,不用which.    　　　　  You can take any room that you like. 

  There is no clothes that fit you here.                            

7.当主句是以which开头的特殊问句时,定语从句关系代词一般用that而不用which.     　　
　Which of the books that had pictures was worth reading? 

8.在强调句型" It is ... that ..." 中,只用that,不用which.    　　
　It is in this room that he was born twenty years ago.

9.在" such (the same) ... as ..." 句型中,关系代词要用as, 而不用which.     　　
　We need such materials as (not which) can bear high temperature. 

10.表示" 正如... 那样 "," 正象..." 之意时, 用as, 不用which.    　　
　 Mary was late again, as had been expected.

11.As 引导的定语从句可以放在主句前面, 也可以放在主句后面或主句中间,而由 which引导的定语从句只能放在主句后面。    　　
　 As we all know, oceans cover more than 70% of the earth.