定语从句的先行词指人的时候就用who,先行词决定关联词的数,如；Do you know the man who is talking with our teacher?先行词指事物的时候就用which,There is the house which I live in.或There is the house in which I live.当然,也可以用that,但是,介词不能用于that之前,如；There is the house that I live in.当先行词中既有人也有事物的时候必须要用that,如；They are talking about the film and the actor that they saw last night.此外,先行词为不定代词的时候也必须要用that引出从句,如；All that I want to know is the fact.(注意,all可以修饰限定或指代可数与不可数名词）
