1. never trouble trouble till trouble troubles you.
麻烦没来找你，就别去自找麻烦。
第一、四个trouble是动词，第二、三个trouble是名词。
2. i think that that that that that student wrote on the blackboard was wrong.
我认为那个学生写在黑板上的那个“that”是错误的。
第一个that是连词，引起宾语从句；第二、五个that是指示代词“那个”；第三个that在这儿相当于名词；第四个that是关系代词，引起定语从句。
3. i know. you know. i know that you know. i know that you know that i know.
我知道。你知道。我知道你知道。我知道你知道我知道。