[Relative Pronouns]

1. 我們通常用 who 來代替『人』； which 來代替『事物』或『動物』，而 that 有時候可以取代 who 或 which。

The girl who is riding a tricycle is my younger sister.

形容詞子句

who is riding a tricycle

Somewhere along the road, there is a shop which sells things from foreign countries.

The girl _______ called you is my friend.  (Who) 打電話給你的女孩是我的朋友

The girl _______ you called is my friend.  (whom) 你電話打給的女孩是我的朋友。

2. 關係代名詞的動詞與先行詞一致

I always buy name brand clothes that are cheap.

The boy who stands there is handsome.

The report which was submitted by the new employee should be revised.

Reservations which were made online can be modified 3 days before checking in.

Reservations which are made online can be modified 3 days before checking in.

3. 關係代名詞的省略

(一) 關係子句中已有主詞，而關係代名詞是作為關係子句的『受詞』，該關係代名詞可以省略。

This is the girl (whom) I saw in the park last night.

I know the singer (whom) you're talking about. 我知道你正在談論的歌手。

(二) 限定用法的關係子句有如下的結構：

The house (which was) painted in red on the hill burnt down last night.

The boy (who is) sitting over there is my son.

4. 關係代名詞不能省略

(一) 非限定性的關係代名詞不能省略。

The car, which I was driving at the time, suddenly caught fire.

(二) 限定性的關係代名詞是關係子句的主詞，且跟著一般動詞

What's the name of the girl who won the 2014 Nobel Peace Prize?

(三) 關係代名詞當受詞用，前面有介系詞，不可以省略。

This is the house in which I was born.

5. 使用關係代名詞，可以把兩個單句合併成一個複句

Laura has a boyfriend. He lives in Japan.

Laura has a boyfriend who lives in Japan.

Laura has a boyfriend. Laura loves him very much.

Laura has a boyfriend whom she loves very much.

Laura has a boyfriend. His house was destroyed in the quake.

Laura has a boyfriend whose house was destroyed in the quake.

Reference

http://www.taiwantestcentral.com/grammar/Title.aspx?ID=248