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兩者都可指物，常可互換。其區別主要在于：
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. 倫敦隊上一個季度打得很好，這個季度卻打得很差。
2. 直接放在介詞后作賓語時，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. 今天早上送來一些波爾圖葡萄酒，為此我得向你道謝。
注：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. 他只有一兩把手槍用來自衛。
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? 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. 你應該做的第一件事是訂個計劃。
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？
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