首先你应该明确的是 when where在定于从句中是关系副词，而which who that则是关系代词。关系副词引导的从句中不应该缺少句子成分e.g:That is a time when I played happily with my friends.
从句：I played happily with my friends中主语是I，谓语是play，宾语是my friends。不缺少成分，又因为先行词是time，所以连接此为when.耳由关系代词连接的定语从句中是缺少句子成分的。e.g: The man who is wearing a hat is our class teacher.从句：who is wearing a hat is our class teacher中只有谓语和宾语，没有主语，故用who.
但是who which that只见又有少许区别who修饰人、which修饰物、that修饰人、物皆可。
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