定语从句,精细(包括只用that,只用which,只用who,不用that的特殊情况)reason后什么时候why或什么时that情况

先行词指物的特殊情况： 一。 必须用that的情况： 1. 先行词被序数词、形容词的最高级修饰时， eg. This is the first job that I have taken up. This is the most interesting novel that I have ever read. 2. 先行词是不定代词,something somebody nothing anything 等 或是 all little much 时， eg. There is so