Question

I think that that that that that boy wrote on the blackboard was wrong

这个句子的第二个that是指示代词还是引导主语从句的？

Answer 1

指示代词

追问

据说可以看成是adj.

追答

指示代词和形容词都是单词的词性，也就是词的性质。那么指示代词和形容词的在句中的地位和作用都可以做定语。所以你可以把这里的第二个指示代词that看作和形容词一样的作用的定语成分。
以下我把每一个that的性质和作用说明以下。

I think that-1 that-2 that-3 that-4 that-5 student wrote on the blackboard was wrong.
为了看的方便，把5个that都依次编上号，下面进行拆解：
1. I think (that-1) that-2 that-3 was wrong （that-1，连接词，后接宾语从句，自身无意义，可省略；that-2，指示代词，“那个”；that-3，指的是写在黑板上的单词“that”）
2. that-2 that-3 that-4 that-5 students wrote on the blackboard was wrong （that-4，连接代词，引导定语从句，代替写在黑板上的单词'that'做定语从句（that student wrote on the blackboard）中wrote的宾语）
3. that-5 student wrote on the blackboard （that-5，指示代词，“那个”学生）

全句意为：我认为那个学生在黑板上写的那个单词（that）是错误的。
这个句子充满娱乐精神，但没有用武之地，于交流无益；如果想表达相同语意，可以改写为：
I think the that which the student wrote on the blackboard was wrong.
限定词the 代替 指示代词 that
连接代词which 代替 连接代词 that
宾语从句连接词that 可省略
终于明白为何上面3种语法概念的存在意义了，就是为了交流无极限

Answer 2

that boy 是一起的，是指示代词