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)

例句:
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.