The book that I like is a novel.

The book which I like is a novel.

The rabbit ate the largest carrot that I've ever seen.

1. All the rabbit that ate a carrot;
2. The only rabbit that ate a carrot;
3. The first rabbit that ate a carrot.

that相对应，which引导从句时，说明先行词并不是唯一的。举例：

The rabbit ate a carrot which I brought.

The rabbit ate a carrot that I brought.

Which carrot was eaten by the rabbit?

The rabbit ate the carrot which was on the table.

The rabbit ate the carrot, which was on the table.

The rabbit ate the carrot, the carrot was on the table.

I made a card for mom who loves me most. （×）

（我给最爱我的那个妈妈做了一张卡片。）

I made a card for mom, who loves me most.

（我给妈妈做了一张卡片，她最爱我。）

My hometown is Beijing that is the capital of China.（×）

（我的家乡是中国首都的北京。）

My hometown is Beijing, which is the capital of China.（√）

（我的家乡是北京，中国的首都。）

My head, which is big, is useful when it rains.

The biggest carrot, which the rabbit ate, was on the table.

The rabbit ate the carrot, which was not surprising.