二动词不定式的构成： to+动词原形

( 1)作主语

动词不定式作主语时，句子的谓语动词常用单数，其位置有以下两种：( 1)把不定式置于句首。

如： To get there by bike will take us half an hour.

( 2)用 it作形式主语，把真正的主语不定式置于句后，常用于下列句式中。

如：

It+ be+名词+ to do It's our duty to take good care of the old.

It takes sb+ some time+ to do How long did it take you to finish the work?

It+ be+形容词+ for sb+ to do It is difficult for us to finish writing the composition in a quarter of an hour.

It+ be+形容词+ of sb+ to do It is stupid of you to write down everything (that) the teacher says.

It seems (appears)+形容词+ to do It seemed impossible to save money.

在句型中，常用表示客观情况的形容词，如： difficult， easy， hard， important， impossible， necessary等;

在句型中，常用 careless， clever， good， foolish， honest， kind， lazy， nice， right， silly， stupid， wise等表示赞扬或批评的词。

在不定式前的 sb，可看作其逻辑主语。这一句式有时相当于 Sb is+形容词+ to do句式，如： It's kind of you to help me with my English.= You are kind to helpme with my English.

( 3)举例

( 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

1、 It's so nice to hear your voice.

听到你的声音真高兴。

2、 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 him 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:

1、 It was silly of us to believe him. 我们真愚蠢，竟然相信了他。

2、 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. 眼见为实。

(错) It is to believe to see.