(3)介词和定语从句的先行词是一种习惯性搭配。 The pen with which he was writing broke. ＝The pen which he was writing with broke. 他写字用的那支钢笔坏了。
(4)表示“整体中的部分”或“所有关系”时，用介词of。 He has three uncles，all of whom are doctors. 他有三个叔叔，他们全都是大夫。
（5)“介词＋关系代词”结构中的关系代词不能用that，但当把介词放在从句有关动词后面时，关系代词可用that，也可省略。
The room (that/which) she lives in is a large one.
她住的是一个大房间。
(6)含有介词的短语动词一般不拆开，介词仍放在动词的后面，因此不能使用“介词＋关系代词”结构。 Is this the watch which he was looking for？
(不能说：Is this the watch for which he was looking？) 这是他正在找的那块手表吗？