介词选择要根据从句的动词来定
eg: I don't know the person with whom you talked just now. 我不认识刚才和你讲话的人.此处从句为 talk with sb. 和某人交谈.用talk with
I don't know the person about whom you talked just now.我不认识你刚才谈论到的那个人.此句中为talk about sb. 谈论某人. 用talk about
当先行词是物的时候用which是,也是考虑从句动词与先行词的搭配
eg. could you find the bus by which you get here.
could you find the bus in/on which you saw the fight.