關係代詞在定語從句中用作介詞賓語時，介詞既可置於從句之首，亦可置於從句之末。但以置於從句之首較為正式。如：

This is the book for which you asked. 這是你所要的書。(關係代詞用作介詞for賓語，介係詞for放在從句的開頭，即which之前)

This is the book which you asked for. 這是你所要的書。(介詞for置於從句之末，which在此可省去)

如果介係詞與動詞或形容片語成固定片語時，介係詞就不可以提前了，如：

The old woman whom he is taking care of is my grandmother. 他正在照顧的那位老人是我的外婆。

關係代詞who和that用作介詞賓語時，介詞須置於句末。如：

The people who you were talking to are Swedes. 你與之談話的那些人是瑞典人。(關係代詞主格who用作介詞to賓語時，介詞to須置於從句之末，who中口語中可省去)

Here is the car that I told you about. 這兒就是我和你談過的那輛汽車。(關係代詞that用作介詞about賓語，介詞about須置於從句之末)

有時從句還有其它成分，介詞則置於從句之中。如：

This is the boy whom he worked with in the office. 那就是與他一道辦公的那個男孩。