不定式主语

1） It's easy (for me) to do that.我做这事太容易了

easy, difficult, hard, important, possible, impossible,comfortable, necessary, better; the first, the next,the last,the best,too much,too little,not enough

It's so nice to hear your voice. 听到你的声音真高兴。
It's necessary for you to lock the car when you do not use it. 当你不用车的时候，锁车是有必要的。

2) It's very kind of you to help us.他帮助我们，他真好。

Kind,nice, stupid,rude, clever,foolish,thoughtful,thoughtless,brave,considerate(考虑周到的),silly, selfish(自私的)

It was silly of us to believe him.我们真愚蠢，竟然相信了他。
It seemed selfish of him not to give them anything.他不给他们任何东西，这显得太自私了。

注意：
1) 其他系动词如，look，appear等也可用于此句型
2) 不定式作为句子成分时，动词用单数形式。
3) 当不定式作主语的句子中又有一个不定式作表语时，不能用It is...to...的句型
(对)To see is to believe.百闻不如一见。
(错)It is to believe to see.

It's for sb.和 It's of sb.

1）for sb. 常用于表示事物的特征特点，表示客观形式的形容词，如easy, hard, difficult, interesting, impossible等：
It's very hard for him to study two languages. 对他来说学两门外语是很难的。

2）of sb的句型一般用表示人物的性格，品德，表示主观感情或态度的形容词，如good, kind, nice, clever, foolish, right。
It's very nice of you to help me. 你来帮助我，你真是太好了。

for 与of 的辨别方法：

用介词后面的代词作主语，用介词前边的形容词作表语，造个句子。如果道理上通顺用of，不通则用for。如：
You are nice.(通顺，所以应用of)。
He is hard.(人是困难的，不通，因此应用for。)