被修飾的詞叫先行詞。
定語從句不同于單詞作定語的情況,它通常只能放在被修飾的詞(即先行詞)之后。定語從句由關(guān)系詞(關(guān)系代詞、關(guān)系副詞)引導(dǎo),關(guān)系代詞、關(guān)系副詞位于定語從句句首。
關(guān)系代詞引導(dǎo)的定語從句
  關(guān)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(dāng)主語、賓語、定語等成分。關(guān)系代詞在定語從句中作主語時,從句謂語動詞的人稱和數(shù)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?他就是想見你的人嗎?(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.? ?(which / that在句中作主語)
? ? ?農(nóng)村出現(xiàn)了前所未有的繁榮。
  The package which / that you are carrying is about to come unwrapped.?(which / that 在句中作賓語)
關(guān)系副詞引導(dǎo)的定語從句
  關(guān)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(guān)系副詞when, where, why的含義相當(dāng)于"介詞+ which"結(jié)構(gòu),因此常常和"介詞+ which"結(jié)構(gòu)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。
  Is this the reason why (for which) he refused our offer? 這就是他拒絕我們幫助他的理由嗎?
2)that代替關(guān)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(dǎo)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。