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：
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在句中作主語）
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）
關系副詞可代替的先行詞是時間、地點或理由的名詞,在從句中作狀語.
1）when,where,why
關系副詞when,where,why的含義相當于"介詞+ which"結構,因此常常和"介詞+ which"結構交替使用,例如：
There are occasions when (on which) one must yield.任何人都有不得不屈服的時候.
Beijing is the place where (in which) I was born.北京是我的出生地.
2）that代替關系副詞
that可以用于表示時間、地點、方式、理由的名詞后取代when,where,why和"介詞+ which"引導的定語從句,在口語中that常被省略,例如：
His father died the year (that / when / in which) he was born.他父親在他出生那年逝世了.
He is unlikely to find the place (that / where / in which) he lived forty years ago.他不大可能找到他四十年前居住過的地方