1.当先行词为all ,few,little,many,much,everything ,something,anything,
none等不定代词时,关系代词一般用that 而不用which/who．
This is all thatIcan find．
Nowadays there are few people that would like to do a thing without pay．
2.当先行词有形容词最高级修饰时,关系代词用that ,而不用which／who．
This is the best dish thatIhave ever tasted．
3.当先行词有表示序数的词如first,second,next ,last等修饰时,关系代词用that,而不用which／who．
This is the last room that we have．
4.当先行词被the only ,the every,the same ,the right等限定词修饰时,关系代词一般用that,而不用which／who．
He is the very man thatI’m looking for．
It is the only book that he left when he died．
5.当先行词既有人又有物时?关系代词只能用that,而不能用which／who．
They said they had received the men and equipment that they needed．
6.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,以避免重复.
Which of the songs that she sang just now is your favourite?
7.在强调句型“ It is ．．．that ．．．” 中,只用that ,不用which．
It was not until4o’clock in the morning that he returned．