先行詞：被定語從句所修飾的詞，通常放在定語從句之前。
引導詞：引導定語從句的詞（也叫關聯詞）
關系代詞：如that, which，who,whom,whose等
關系副詞:如when, where, why等）。
引導詞通常位於先行詞和定語從句之間，它既起聯系作用，又充當從句中的某個句子成分。
其間主要區別：
that與which的用法區別
兩者都可指物，常可互換。其區別主要在於：
1. 引導非限制性定語從句時，通常要用which：
The current, which is very rapid, makes the river dangerous. 水流湍急，使這條河很危險。
He has to work on Sundays, which he doesn’t like. 他得在星期天工作，他是不喜歡這樣的。
The London team, which played so well last season, has done badly this season. 倫敦隊上壹個季度打得很好，這個季度卻打得很差。
2. 直接放在介詞後作賓語時，通常要用which：
She may be late, in which case we ought to wait for her. 她可能晚到，那樣我們就要等等她。
The documents for which they were searching have been recovered. 他們找尋的文件已找到了。
This morning some port wine came, for which I have to thank you. 今天早上送來壹些波爾圖葡萄酒，為此我得向妳道謝。
註：有時“介詞+which”引導的定語從句可以轉換成“介詞+which+不定式”結構：
He had only the long nights in which he could study. =He had only the long nights in which to study. 他只有漫漫長夜可用來學習。
He had a couple of revolvers with which he could defend himself.= He had a couple of revolvers with which to defend himself. 他只有壹兩把手槍用來自衛。
3. 當先行詞是下列不定代詞或被它們修飾時much, little, none, all, few, every(thing), any(thing), no(thing) 等時，通常用that：
All that she lacked was training. 她缺的只是訓練。
Have you everything that you need? 妳需要的東西都有了嗎?
The sleeping man’s subconscious mind retained everything that was said around him. 這位酣睡的人頭腦的下意識能記住他周圍的人說的話。
She would never do anything that was not approved of by her parents. 她父母不贊同的事她絕不會做。
4. 當先行詞有the very, the only, the same 等修飾時，通常用that：
This is the only example that I know. 我知道的例子只有這壹個。
Those are the very words that he used. 那是他的原話。
5. 當先行詞有形容詞最高級或序數詞(包括last, next等)等修飾時，通常用that：
This is the best dictionary that I’ve ever used. 這是我用過的最好的詞典。
The first thing that you should do is to work out a plan. 妳應該做的第壹件事是訂個計劃。
6. 當關系代詞在定語從句中用作表語時，通常用that：
China is not the country (that) it was. 中國已不是過去的中國了。
7. 當先行詞是壹個既指人又指物的並列詞組時，通常用that：
They talked about the persons and things that most impressed them. 他們談論了使他們印象最深的人和事。
8. 當要避免重復時：
Which is the course that we are to take? 我們選哪門課程？
that與who的用法區別
1. 兩者均可指人，有時可互換（包括當先行詞為 all, anyone, someone等，同時也包括先行詞受the only等的修飾時）：
All that [who] heard him were delighted. 所有聽了他講話的人都很高興。
He is the only one among us that [who] knows Russian.他是我們中間唯壹懂俄語的人。
Anyone who [that] is over sixteen is allowed in. 任何超過十六歲的人都允許進去。
It is impolite to turn your back on someone who [that] is speaking to you. 人家跟妳說話，妳把背朝人是不禮貌的。
2. 但是在下列情況，通常要用 that：
①當先行詞是壹個既指人又指物的並列詞組時：
I made a speech on the men and things that I had seen abroad. 我就我在國外所見到的人和事作了報告。
②當先行詞是who時(為避免重復)：
Who was it that won the World Cup in 1982? 誰贏得了1982年的世界杯？
③當關系代詞在定語從句中作表語時(可省略)：
Tom is not the boy (that) he was. 湯姆這孩子已不是以前那個樣子了。
3. 但是在下列情況，通常要用 who：(www.yygrammar.com)
當先行詞為 those 且指人時，英語習慣上要用 who 來引導定語從句。如：
Those who do not wish to go need not go. 不願去的人不需要去。
Those who couldn’t walk were carried on stretchers. 那些不能走的人都用擔架擡著。
It is said that those who eat the most are the least healthy. 據說吃得最多的人身體最差。
Only those who had booked in advance were allowed in. 只有那些預先訂票的人可以進去。
The plan was supported by those who wished to listen to more good English. 這計劃受到想多聽好英語的人的支持。
Let us spend a few moments in silence so that / in order that we remember those who died to preserve our freedom. 讓我們默哀片刻，以緬懷那些為維護我們的自由而犧牲了的人們。
註：偶爾也可見到在 who 之後用 that 引導定語從句的用例，但相當少見。如：
The danger is that people who pay their bills on time will be lumped in with those that don’t. 危險的是那些及時付賬的人將會同那些不及時付賬的人混在壹起。（摘自《朗文當代英語辭典》2003年第4版）