which与that两者都可指物,常可互换.其区别主要在于：
1.引导非限制性定语从句时,通常要用which.如：She received an invitation from her boss,which came as a surprise.她收到了老板的邀请,这是她意想不到的.
2.直接放在介词后作宾语时,通常要用which.如：The tool with which he is working is called a hammer.他干活用的那个工具叫做锤子.
3.much,little,none,all,few,every(thing),any(thing),no(thing) 等时,通常用that.如：There was little that the enemy could do but surrender.敌人无法,只有投降了.All [Everything] that can be done must be done.凡能that can be done must be done.
4.当先行词有the very,the only,the same 等修饰时,通常用that.如：This is the only example that I know.我知道的例子只有这一个.Those are the very words that he used.那是他的原话.
5.当先行词有形容词最高级或序数词(包括last,next等)等修饰时,通常用that.如：.如：This is the best dictionary that I’ve ever used.这是我用过的最好的词典.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.
6.当关系代词在定语从句中用作表语时,通常用that.如：China is not the country (that) it was.中国已不是过去的中国了.
7.当先行词是一个既指人又指物的并列词组时,通常用that.如：They talked about the persons and things that most impressed them.他们谈论了使他们印象最深的人和事.他们谈论了使他们印象最深的人和事.
8.当要避免重复时.如：Which is the course that we are to take?我们选哪门课程?