關係代名詞有who、whom、whose、which、that可以用來代替出現過的先行詞
<例>The boy who is riding a bicycle is my bother.
上面例句關代who代替先前出現的boy且也引導了who is riding a bicycle
來修飾代替的先行詞boy且具有連接詞功能
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
She has a dog which barks loudly他有一隻狗它叫得很大聲
關代which一方面它代替先前出現的名詞 dog，另一方面引導關係子句"which barks loudly"
來修飾所替代的先行詞 "dog"
I like the watch which my mother gave me.
I like the watch that my mother gave me.
上面兩句例句相同that可代替which，關代which、that代替watch
若句子中間遇到"，"逗號則不能使用that代替
I always buy name brand clothes that are cheap.
我總是買便宜的名牌衣服。
先行詞 clothes 是複數
關代that 用複數動詞 are

關代的省略:
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.
這是我昨晚在公園裡看到的女孩
關係子句中已有主詞 I，且關代whom 是動詞saw 的受詞，這種情形 whom可以省略
The girl (who is) sitting over there is my daughter.
坐在那兒的女孩是我的女兒
"who is" 可以省略
The girl sitting over there is my daughter.