一、应该根据与先行词或者定语从句中某个词语的搭配关系使用适当的介词。例如：

The foreign expert with whom we’ve worked together for two years is returning home next week. 与我们一起工作了2年的那位外国专家下星期就回国了。（赵俊英：《现代英语语法》p.188）

【说明】（1）因为work with sb.表示“与某人一起工作”，所以，关系代词前面的介词应该用 with ; (2) 汉语译文应该改为：“我们与之一起工作了2年的那位外国专家下星期就回国了。”如果汉语译文不变，英语应该是：The foreign expert who has worked together with us for two years is returning home next week.

Is this the comrade with whom you worked five years ago ? 这就是5年前你与之一起工作过的那位同志吗？ (赵俊英：《现代英语语法》，p.1297)

This is the camera with which he often takes photos. 这就是他经常用来拍照的相机。

【说明】这里的先行词 camera 与  with 构成搭配关系。He often takes photos with the camera. 他经常用这台相机来拍照。

This is the book from which I have learned a lot.  这就是我从中学到很多东西的那本书。

【说明】这里的搭配关系是：learn sth. from ...

二、在介词后面的关系代词只能用 whom或者 which; 而且，这种关系代词不能省略。如果把介词放在定语从句中其它的位置上，关系代词可以用 who, whom, which 或者 that，也可以省略。例如：

The friend with whom I was travelling spoke English.

= The friend (who / that / whom) I was travelling with spoke English.

我和他一起旅游的那位朋友讲英语。

This is the man to whom I gave the book.

= This is the man  (who / that / whom) I gave the book to.

这就是我把书给他的那个人。

三、如果“动词 + 介词”属于固定搭配的短语动词，通常不能把其中的介词放在关系代词前面。如：

 【正确】He is the patient (who / that / whom) I am taking care of.

他就是我照顾的病人。

 【错误】He is the patient of whom I am taking care.