在定語從句中這兩個詞有可互換的時候.但that用的多一些.如果先行詞為下面的情況就用that
①當先行詞是一個既指人又指物的并列詞組時：
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(fù))：
Who was it that won the World Cup in 1982?誰贏得了1982年的世界杯?
③當關(guān)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.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(xùn)練.
Have you everything that you need?你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一個.
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(shù)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.你應(yīng)該做的第一件事是訂個計劃.
6.當關(guān)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.當先行詞是一個既指人又指物的并列詞組時,通常用that：
They talked about the persons and things that most impressed them.他們談?wù)摿耸顾麄冇∠笞钌畹娜撕褪?
8.當要避免重復(fù)時：
下面情況是只用which
1.在非限制性定語從句中只能使用關(guān)系詞which,不能使用關(guān)系詞that
2.在“介詞+關(guān)系代詞”引導(dǎo)的定語從句中,只能使用關(guān)系詞which,不能使用that