①依据定语从句中动词或形容词等所需要的某种习惯搭配来确定。

I bought a great many books, on which I spent all my money that I saved. 

我买了很多书，这些书花了我积攒的所有钱。（on依据spend...on...确定的）

② 依据与先行词搭配的具体意义而定。

Ill never forget the time during which I spent my childhood in the country. 

我永远不会忘记我在乡村度过的时光。

③ 根据所表达的意思来确定。

The colorless gas without which we cannot live is called oxygen.

这种无色的、离了它我们不能活的气体就是氧气。

★★注意：有些“动词＋介词”习语如look for, look after, call on等如果介词提前将会失去动词习语的意义，所以不可把介词置于关系代词前。