which在什么情况下用？
1、表示地点，时间和原因的“介词＋which”分别相当于where，when，why。
The earth on which／where we live is a planet．
I'll never forget the day on which／when I joined the League．
2、way后常用that代替in which，也可省略that。
That was the way in which the old lady looked after us．
3、“of＋which”起形容词的作用，相当于whose（用来指物），其词序通常是“n．＋of which”。
They live in a house whose door／the door of which opens to the south．
介词的选用原则：
1、根据定语从句中谓语动词的习惯搭配
This is the book on which I spent $ 8.
This is the book for which I paid $ 8.
2、根据先行词的搭配习惯
I remember the days during which I lived there.
I remember the day on which I graduated from university.
3、根据整个句子所表达的意思来决定
The colorless gas without which we can’t live is called oxygen.
4、英语中为了强调某一名词，不定式前面也可以加上关系代词。
Here is the money with which to buy the piano.
She is the right person on whom to depend
