at which,in which,on which,for which,by which,from which 分别等

前提是在定语从句中,关键要讨论先行词是时间还是地点.但是有些引导词是不能替换的：例如：by which,from which----因为这些介词本身就有意义了,就不能像when、where这样只替代先行词的意义了.
能替代的,可以互换的往往是前几个常见的：例如--at which = when、where