Did you see the letter that [which] came today? 你看到今天来的那封信了吗?
This is the book which [that] I told you about. 这就是我曾跟你谈起过的那本书。
但要注意，下列情况一般不宜换用：
1. 通常只用 which 的场合:
(1) 引导非限制性定语从句时:
I lost my watch, which is not found yet. 我把表丢了，到现在还没有找到。
He changed his mind again, which made us all angry. 他又改变了主意，这使我们大家很生气。
(2) 直接放在介词后作宾语时:
This is the book of which I was speaking. 这本书就是我跟你们提到过的那本书。
The tool with which he is working is called a hammer. 他干活用的那个工具叫做锤子。
2. 通常要用 that 的场合(偶尔也可能用 which, 但较少见)：
(1) 当先行词是下列不定代词或被它们修饰时：much, little, none, all, few, every(thing)，any(thing)，no(thing)等。如：
All that glitters is not gold. 发光的不一定都是金子。
I never took anything that didn’t belong to me. 我从来不拿不属于我的东西。
All [Everything] that can be done must be done. 凡能做的事都必须做。
(2) 当先行词有the very, the only, the same 等修饰时。如：
Those are the very words that he used. 那是他的原话。
This is the only example that I know. 我知道的例子只有这一个。
(3) 当先行词有形容词最高级或序数词(包括last, next等)等修饰时。如：
This is the most interesting story that I’ve ever read. 这是我所读过的最有趣的故事。
The first thing that we should do is to work out a plan. 我们应该做的第一件事是订个计划。
(4) 当关系代词在定语从句中用作表语时。如：
China is not the country (that) it was. 中国已不是过去的中国了。
(5) 当先行词是一个既指人又指物的并列词组时。如：
The train ran over a boy and his dog that were justcrossing the track. 火车压死了正穿越轨道的一个男孩和他的狗。
(6) 当要避免重复时。如：
Which is the car that overtook us? 超过我们的是哪辆车?
