(1)在非限制性定语从句中，不能用 that;只能用 which 指物、事，who/ whom 指人，when 指时间，where 指地点;

　　(2)在由“介词+关系代词”引导的定语从句中，只能用 which 指物，whom 指人;若“介词+which”结构在从句中充当时间、地点状语时，可以同关系副词 when, where 互换，其中介词的选择，需根据与先行词的搭配或从句谓语动词的搭配确定;why 只能用 for which;

　　(3)限制性定语从句中，名词前有 such 和 the same 修饰时，关系代词用 as;在非限制性定语从句中，as 作为关系代词可以代替整个主句，位于句首或句中，而 which 只能位于句中。

　　如：Is he the man who / that wants to see you?(who / that 在从句中作主语)

　　The package (which / that) you are carrying is about to come unwrapped.(which / that 在句中作宾语)

　　They rushed over to help the man whose car had broken down.(whose 在从句中作定语)

　　Beijing is the place where (=in which) I was born.(where 在从句中作地点状语)

　　Water, which is a clear liquid, has many uses.

　　(which 引导非限制性定语从句，有逗号隔开)

　　As might be expected, John was admitted to the university.

　　(as 引导非限制性定语从句，可位于句首)

　　(4)只用 that 的情况：

　　①先行词为 all, everything, anything, nothing, little, much 等不定代词时;

　　②先行词被 all, any, every, each, much, little, no, some, few 等修饰时;③先行词有形容词最高级和序数词修饰时;④先行词既指人又指物时;⑤先行词被 the only, the very 修饰时等。