划分下英语句子语法结构。尤其是to answer。

Question

That question is difficult to answer.
刚查了百度百科，作状语，表程度。不过好像说不过去，to answer哪里是程度呢。见百度百科非谓语动词。
刚查到了，是作difficult的补足成分，叫做不定式作形容词的补足成分。

Answer 1

That question 是主语 is 是系动词 difficult 是表语 to answer是补语----这么分析不合理
应该是： That question 是主语 difficult 是标语 to answer 是后置定语（不定式作定语）
意思是：这个问题很难回答。

Answer 2

that question主语 is系动词 difficult表语 to answer为原因状语

Answer 3

That question 是主语 is 是系动词 difficult 是表语 to answer是补语

Answer 4

That question is difficult 是整个句子 to answer.是修饰语

Answer 5

这是动词不定式作后置定语：这个问题很难回答。