that與which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。如：She received an invitation from her boss, which came as a surprise. 她收到了老板的邀請，這是她意想不到的。
　　2. 直接放在介詞后作賓語時，通常要用which。如：The tool with which he is working is called a hammer. 他干活用的那個工具叫做錘子。
　　3. much, little, none, all, few, every(thing), any(thing), no(thing) 等時，通常用that。如：There was little that the enemy could do but surrender. 敵人無法,只有投降了。All [Everything] that can be done must be done. 凡能that can be done must be done.
　　4. 當先行詞有the very, the only, the same 等修飾時，通常用that。如：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。如：。如：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。如：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。如：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。他們談論了使他們印象最深的人和事。
　　8. 當要避免重復時。如：Which is the course that we are to take? 我們選哪門課程？