介詞后跟有關系代詞時要注意：此時的關系代詞雖然用作介詞的賓語，但不能省略，可以是which，但不能是that。另外，某些表示時間、地點或原因的“介詞+關系代詞”結構可以與相應的關系副詞when, where, why 互換。例如：

　　This is the pan in which I boiled the milk. 這是我煮牛奶的鍋。

　　I wanted to find someone with whom I could discuss music. 我想找一個能和我談音樂的人。

　　The documents for which they were searching have been recovered. 他們找尋的文件已經找到了。

　　These are the reasons for which [why] we do it. 這些就是我們這樣做的理由。

　　The period in which [when] man learnt to make tools of iron is called the Iron Age. 人類學會制造鐵器的時期稱為鐵器時代。

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