8.It is kind of you to help me with my English.

　　It代指of you to help me with my English（这个句子已经转化成了名词词组）这件事。

　　of引出 help me with my English的主体you，to转化含五动全形help的部分为to help me with my English的非五动全形名词词组。

　　用of 而不用for，因为kind 属于人（you）的属性特征，of 表“you拥有kind的属性特征”。

　　换言之（比较例句7和例句8），可以说you are kind，而不能说he is difficult.

　　9.They feel it difficult to do the math.

　　他们觉得难解这道数学题。

　　10.I find it difficult for him to do the math.

　　我感觉他难解这道数学题。

　　11.We think it a question whether he’ll come or not.

　　我们认为他来还是不来还是未知数。

　　12.The teacher remains it undecided who will be the leader of the class.

　　老师仍然未决定谁来当班长。

　　13.I took it for granted that you would help me with my English.

　　我曾想当然地以为你会帮我学习英语。

　　14.He found it no use arguing with his mother about doing his homework on time.

　　他发觉跟他妈妈争论准时完成家庭作业的问题没有用。

　　15.I’d much appreciate it if you could help me with my English.

　　如果你帮我学习英语，我会非常感激。

　　以上句9～句15，it指代部分都用斜体表示。与句1～句8的区别在于：句9～句15中的it做受体，结构作用不同而已。其它解释一样。