定语从句是一种用句子作定语修饰名词的主从复合句，通常直接跟在被修饰的名词后面。被修饰的名词称为先行词。定语从句一般由关系代词引导。关系代词连接定语从句和先行词，本身又在定语从句中充当某个成分。例如：

I know the man who spoke to you just now.

先行词 关系代词(who)在定语从句中作主语

我认识刚才和你说话的那个人。

1.关系代词who(m), which,that

that引导的定语从句

that作关系代词时可以修饰表示人或物的名词，作宾语时可以省略。例如：

The girl that is sitting under the tree is a good runner.

坐在树下的那个姑娘是很好的赛跑运动员。

The picture that hangs on the wall was painted by him.

挂在墙上的画是由他画的。

The cartoons (that) they have made are always welcomed by the children.

他们制作的动画片总是受儿童欢迎。

Finally, the thief handed everything that he had stolen to the police.

那贼最终把偷的全部东西交给了警察。

The best that I could do was to apologize.

我唯一能做的就是赔礼道歉了。

This is the most interesting story bookthat I have ever read.

这是我读过的一本最有趣的故事书。

2.必须用that作关系代词的情况

当定语从句先行词含形容词最高级，或者表示序列意义的形容词以及only时，关系代词必须用that。例如：

This is the best film (that) I have ever seen.

这是我看过的最好的电影。

The best (that) we can do is try to stay cool and muddle through.

我们能做的顶多就是设法保持冷静，蒙混过关。

That was the only science fiction film (that)I was interested in.

那是唯一一部我感兴趣的科幻片。

The first place (that) I visited in Beijing was the Great Wall.

我在北京参观的第一个地方是长城。

This is the longest (that) I've ever seen.

这是我见过的最长的（某物）。

He is the cleverest student that I have seen.

他是我见过的最聪明的学生.