“宾语从句的连接代词”的定义是什么？为啥“where”不能做“宾语从句的连接代词”？

where在哪里都是副词。宾语从句的连接代词指可以引导宾语从句的疑问代词，其在宾语从句中做主语或宾语，表语等的which what 或不做任何成分的that。其实宾语从句无连接代词一说，因为它和主句无关，准确的说应该叫宾语从句的引导词。

定语从句中的关系代词的作用是代替先行词,在从句中作先行词的成分,所以先行词在从句中作什么成分关系代词就作什么成分,如
The book that /which I bought yesterday is very interesting.(关系代词作宾语)
The book that /which was written by him is very interesting.(关系代词作主语)
The boy who is standing at the door is our monitor.(关系代词作主语)
The man whom you were talking with just now is Tom's father.(关系代词作宾语)主语从句，即在复合句中充当主语成分的句子，例如"That he finished writing the composition in such a short time surprised us all"，他在很短的时间内写完作文让我们都很惊讶，此复合句主要描述的是某事让我们惊讶，某事就是该复合句的主语，因此“That he finished writing the composition in such a short time”便是该复合句的主语从句。
引导主语从句的连词主要有：
从属连词：that whether
连接代词：who whoever whom whose what whatever which whichever
连接副词：when where how why

因为where是副词，只能作连接副词

where可以引导宾语从句。