定語從句(Attributive Clauses)在句中做定語，修飾一個名詞或代詞，被修飾的名詞,詞組或代詞即先行詞。定語從句通常出現在先行詞之后，由關系詞(關系代詞或關系副詞)引出。

關系代詞有：who, whom, whose, that, which等。

關系副詞有：when, where, why等。

關系代詞引導的定語從句

關系代詞所代替的先行詞是人或物的名詞或代詞，并在句中充當主語、賓語、定語等成分。關系代詞在定語從句中作主語時，從句謂語動詞的人稱和數要和先行詞保持一致。

1)who, whom, that

這些詞代替的先行詞是人的名詞或代詞，在從句中所起作用如下：

Is he the man who/that wants to see you?

他就是你想見的人嗎?(who/that在從句中作主語)

He is the man whom/ that I saw yesterday.

他就是我昨天見的那個人。(whom/that在從句中作賓語)

2) Whose 用來指人或物，(只用作定語, 若指物，它還可以同of which互換), 例如：

They rushed over to help the man whose car had broken down. 那人車壞了，大家都跑過去幫忙。

Please pass me the book whose (of which) cover is green.請遞給我那本綠皮的書。

3)which, that

它們所代替的先行詞是事物的名詞或代詞，在從句中可作主語、賓語等，例如：

A prosperity which / that had never been seen before appears in the countryside. 農村出現了前所未有的繁榮。(which / that在句中作賓語)

The package (which / that) you are carrying is about to come unwrapped. 你拿的包快散了。(which / that在句中作賓語)