一、关系代词直接放在介词后面时,要用which.例：
Read the passage to which I referred in my talk.
读一下我在演讲中提到过的那段文章.
His new car,for which he paid ￡7000,has already had to be repaired.
他花700英镑买来的那辆新车已经不得不修理了.
二.which还可以用作关系限定词,后跟名词,that则不能这样用.例：
He changed his mind for the second time,after which I refused ever to go out with him again.
他第二次变卦,以后我再也不跟他出去了.
The postman comes at 6.30 in the morning,at which time I am usually fast asleep.
邮递员早晨六点半就到,而这时我通常还熟睡着呢.
He may come,in which case I'll ask him.
他也许会来,来了我就问他.
There are 2 left,one of which is almost finished and the other of which is not quite.
剩下两件事,一件快做完了,另一件还有得做.
三、引导非限制性定词从句时,一般要用which,不用that.例：
Football,which is a popular game,is played all over the wofld.
全世界都踢足球,它是一项很受欢迎的运动.
She spent all evening talking about her latest book,which none of us had ever heard of.
她花了整个晚上谈她最近的新书,关于这本书,我们没有一个人听说过.
