定語從句是非常重要的。鑑於它的重要，將定語從句分成四部分進行探討。
一、定語從句的概述：
一個句子作定語，用來修飾限定名詞。例如：
Law is a discipline which encourages responsible judgment.
全句為主句，discipline名詞，用which encourages responsible judgment.修飾。
注意：定語從句可以修飾所有名詞。被修飾的名詞稱為先行詞，因為定語從句所修飾的名詞一定放在從句前面，用關係詞連接，且關係詞與先行詞是同等的。例如上面的句子可分解為：Law is a discipline. A discipline encourages responsible judgment.若將兩句話構建成一句話，那麼出現有重複的名詞，在英語中我們就用一定的詞來替換有重複的詞。如是替換人則用who，替換物則用which。用來替換的關係詞是要作句子成分的，故一定一定不能省略。
二、定語從句的寫法
先行詞n. + 關係詞 + 陳述句（定語從句）
關係詞選擇看先行詞是表示什麼。
注意：whom是專門作賓語的。先行詞是事/物，關係詞選擇which/thatTeachers need to be aware of the emotional,intellectual,and physical changes that young adults experience.
changes先行詞（表示的是變化這件「事」）故用了that
Furthermore,the legal system and the events which occur within it are primary subjects for journalists.
events先行詞（表示事件）用which修飾。
2、先行詞是人，關係詞選擇who/whom/that
who/that只要是指人，都能通用，但是whom只能用做賓語。
Most archaeological sites,however,are discovered by archaeologists who have set out to look for them.
archaeologists（先行詞表示人的）故用who作關係詞。
Retailers that master the intricacies of wholesaling in Europe may well expect to rake in substantial profits thereby.
Retailers（先行詞表示人物的）用that作關係詞。that master the intricacies of wholesaling in Europe陳述句作從句。
3、先行詞是人/物，表示人或物的所有關係（即XX的），關係詞選擇whose。
On display here are various fantasy elements whose reference,at some basic level,seems to be the natural world.
這個句子倒裝過的，順序排列應該是：Various fantasy elements are on display here...
之所以倒裝是因為後面有定語從句修飾various fantasy elements，如果按順序排列的話使句子看起來頭重腳輕。「這些元素的參考」XX的所以用whose關係詞。
This, for those as yet unaware of such a disadvantage,refers to discrimination against those whose surnames begin with a letter in the lower half of the alphabet.
4、先行詞是時間，關係詞選擇when
It is difficult to the point of impossibility for the average reader under the age of forty to imagine a time when high-quality arts criticism could be found in most big-city newspapers.
5、先行詞是地點，關係詞選擇where
In dreams, a window opens into a world where logic is suspended and dead people speak.
6、先行詞是原因，關係詞選擇why
The other reason why costs are so high is the restrictive guild-like ownership structure of the business.
重點關注前兩種定語從句。
關於定語從句的關係詞的幾點注意：1、定語從句的關係詞每一個都必須作成分，包括that。
2、whom指人，只能在做賓語時使用。
3、when/where/why引導定語從句時，前面的先行詞必須與之匹配。
4、關係詞在定語從句中作賓語時，都可以省略。無論及物動詞後（動賓）還是介詞後（介賓），均可以省略。
5、關係詞一定在從句句首，但是它表示的成分不一定在句首。
Are humans actually aware of the world they live in?
可以在they live in前面加上關係詞which，但實際它表示的成分在live in後作賓語。
These rules say they must value some assets at the price a third party would pay,...
a third party would pay可以是which a third party would pay,但是which所表示的成分應該在pay後面作賓語。
注意：特殊的關係詞whereby = by which（近15年出現過2次）
The definition also excludes the majority of teachers,despite the fact that teaching has traditionally been the method whereby many intellectuals earn their living.
可以將whereby換成by which。
今天我們就討論在這裡，下次我們探討定語從句的分類。