【问】 《实用英语语法》(张道真编著，外研社，1995，P.96)说：
 “which代表事物，在从句中可以作主语或宾语……that代表事物的时候更多一些，也可代表人，在从句中可以作主语、宾语或表语。”
 据此我得出如下结论：
 当中心词为表物的名词，而关系代词在从句中作主语或宾语时，which和that是完全可以互换的。
 【答】 《实用英语语法》，作为一部中级英语语法书，对which和that所作的陈述是恰当的。你如果是一位英语初学者的话，那你所做的结论是很实用的。但是，你如果具有较高的英语水平，则应查找其他资料，进一步弄清which和that的惯用法问题。下面提供一些参考资料。
 通常使用that的场合：
 A.当先行词是all，anything，something，everything，nothing，little，much，none，one时，人们大都喜欢使用that或省略充当宾语的that(即零关系代词)：
 1. All is well that ends well.
 结果好，一切都好。
 2. Have you everything that you need? (Eckersley)
 你是否(拥)有你所需要的一切东西?
 3. She has everything (that) a woman could wish for. (GPU)
 她拥有女人想获得的一切。
 4. I'll tell you something that you didn't know before. (EED)
 我要告诉你你从前不知道的一些事情。
 5. I never have taken anything that didn't belong to me. (Galsworthy)
 我从不去获取不属于我的东西。
 6. Much that has been said tonight will soon be forgotten. (CGEL)
 今天晚上说的许多话很快就会忘记。
 7. There was little that interested him at the motor show.
 汽车展销会上引起他兴趣的东西极少。
 8. The government has promised to do all that lies in its power to help.
 政府答应尽全力进行援助。
 B.当被修饰的名词(即先行词)有only，all，any，few，little，no，one of等修饰语时，人们通
 常喜欢用that或者省略that(当that作宾语时)：
 The only thing that impresses me is her directness.
 唯一给我留下深刻印象的就是她的直率。
 C.当先行词受最高级形容词、顺序数词、last、next修饰时，通常使用that或零形式：
 They eat the finest food that is available. (GCE)
 他们吃所能买到的最好的食品。
 D.当先行词既含表人的名词又含表物的名词时，这时只能使用that：
 The generals complained that the Defence Department had not sent the extra men and e-quipment that they needed. (CGEL)
 将军们抱怨国防部没有给增派他们所需要的人员和设备。
 通常使用which的场合：
 A.当关系代词之前有介词时，只能使用which：
 1. Here is the car about which I told you. (Eckersley)
 这就是我向你谈到过的那辆汽车。
 2. The documents for which they were searching have been recovered. (Chambers)
 他们在寻找的文件已经找回来。
 3. This is a subject about which we might argue for a long while. (Sinclair)
 这是一个我们可能要争论很长时间的问题。
 但是，当介词位于句末时，则可以使用that：
 1. This is somethingI cannot put up with.
 这是我不能忍受的一件事。
 2. the office that he works at
 他工作的办公室
 3. The chair that you are sitting on is an antique. (Chambers)
 你坐着的那把椅子是一件古物。
 B.当关系代词之后有插入成份时：
 I have received your courteous and constructive letter which，as I have told you，was ur-gently needed by the press.
 我已收到您那封彬彬有礼而富有建设性的来信。这封信，正如我所告诉您的，是新闻界所急需的。