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.
used to/be used to的分别 
　  used to + do："过去常常"表示过去习惯性的动作或状态，但如今已不存在。
　　Mother used not to be so forgetful.　　 
　　Scarf used to take a walk.　 (过去常常散步)
      be used to + doing： 对……已感到习惯，或"习惯于"，to是介词，后需加名词或动名词。
　　He is used to a vegetarian diet.　　 
　　Scarf is used to taking a walk.(现在习惯于散步)
used to 的用法
否定式简写为usedn't） 过去经常，以前常常
We used to go there every year.  （我每年都去那儿。）
He is not what he used to be. 
他已不是旧日的他了。
我们公司过去和他们的公司常有业务往来。
This used to be a shabby house.  （此房年久失修）
used to,would这两个词语都可以表示过去常做某事，有时可以换用。
used to do 强调整过去习惯性的行为或状态，但是现在没有这种行为或状态了。因此，这个短语的内涵是今昔对比。