关系代词who, which, that 的考点：

I know the girl. The girl works in IBM.

I know the girl who / that works in IBM.

The old man is a famous artist. We visited the old man yesterday.

The old man who /that we visited yesterday is a famous artist.

The laptop is made by DELL. I bought the laptop last week.

The laptop which / that I bought last week is made by DELL.

先行词为以下情况，关系代词必须用that:

1.先行词被形容词最高级修饰：

He is the hardest-working student that I have ever taught.

2.先行词被序数词修饰：

He is the first man that finished the work on time.

3.先行词被only, just, the same, next等词修饰：

This is the only dictionary that I have at hand.

4.先行词本身为不定代词：

Is there anything that I can do for you?

5.先行词中既包含人又包含物：

We know nothing about the doctors and the hospitals that you are talking about.

6.正式用法中，先行词是动物的时候

We have to protect whales and sharks that are being threatened by death.