介詞後跟有關係代詞時要注意：此時的關係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+關係代詞”結構可以與相應的關係副詞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. 他們找尋的檔案已經找到了。
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. 人類學會製造鐵器的時期稱為鐵器時代。
