从句中如果是介宾的话,介词与先行词放在从句前,这时引导词不可省略,而且引导词只能用which,不能用that.
先行词是形容词的最高级,序数词,only.last 或者very,这时候只能用that.
当先行词本身是不定代词：anything,nothing,all,little,much,这时只能用that 来引导.
先行词是一个既有人又有物的词组时,这时候只能用that 来引导.
当主句的疑问词时Who时,定于从句只能用that来引导.
比如：Who's the man that is smoking over there.