The two things about which he was not sure were the grammar and the spelling.

＝The two things which he was not sure about were the grammar and the spelling.

他在语法和拼写这两方面还是没太有把握。

3．介词与定语从句的先行词是一种习惯性搭配

The pencil with which he was writing broke.

＝The pencil which he was writing with broke.

他写字用的那支铅笔坏了。

4．表示"所有关系"或"整体的部分"时，用介词of

He lives in a room，the window of which faces west.

他住在一个房间里，房间的窗子面朝西。

He has three uncles，all of whom are doctors.

他有三个叔叔，他们全都是医生。

5．在"介词＋关系代词"的结构中，关系代词不能用that；但当把介词放在从句中的有关动词之后时，关系代词可用that，也可以省略

The room in which she lives is a large one.

＝The room (that/which) she lives in is a large one.

她住的房间是个大房间。

6．有些"动词＋介词"短语如look for，look after，call on等，如果介词提前将会失去动词短语的意义，所以不可把介词置于关系代词前

(正)Is this the watch which he was looking for?

(误)Is this the watch for which he was looking?

这是他正在找的那块手表吗？

(正)This is the baby (whom/that) my grandma takes care of.

(误)This is the baby of whom my grandma takes care.

这就是我奶奶照看的婴儿。

Ⅰ.单句语法填空

1．Many young people，most of whom were well­educated，headed for remote regions to chase their dreams.(2016·江苏)