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。