一、that 与 which
两者都可指物，常可互换，其区别是：
(1) 引导非限制性定语从句或直接放在介词后作宾语时，要用which。如：
Peter drove too fast, which was dangerous. 彼得开车很快，这是很危险的。
This is a subject about which we might argue for a long while. 这是一个我们可能长时间争论的问题。
(2) 当先行词为不定代词或被其修饰、受the very, the only, the same 等修饰、受形容词最高级或序数词(包括last, next)等修饰、关系代词在定语从句中用作表语或先行词是一个既指人又指物的并列词组时，通常用that。如：
All that can be done must be done. 凡能做的事都必须做。
This is the only book (that) I find useful. 这是我发现的惟一有用的书。
The first thing that you should do is to work out a plan. 你应该做的第一件事是订个计划。
China is not the country (that) it was. 中国已不是过去的中国了。
2. as 与 which
(1) 引导限制性定语从句时，在 such, as, the same 后只能用 as，其他情况用 which。如：
He is not such a fool as he looks. 他并不像他看起来那么傻。
He smokes as expensive a cigarette as he can afford. 他抽一种他买得起的最贵的烟。
(2) 引导非限制性定语从句时，有时两者可互换，但若位于主句之前，则只能用as。如：
I live a long way from work, as [which] you know. 我住得离工作单位很远，这你是知道的。
As is known to everybody, the moon travels round the earth once every month. 月球每月绕地球转一周，这是每个人都清楚的。
另外，as 引导的非限制性定语从句应与主句在意义上和谐一致，which无此限制。如：
He went abroad, as [which] was expected. 他出国了，这是大家预料到的。
He went abroad, which was unexpected. 他出国了，这让大家感到很意外。(不用as)