動(dòng)詞不定式做主語
動(dòng)詞不定式作主語時(shí)，句子的謂語動(dòng)詞常用單數(shù)，其位置有以下兩種：
(1)把不定式置于句首。如：
To get there by bike will take us half an hour.騎自行車到那兒將花費(fèi)我們半個(gè)小時(shí)。
To make up for lost time is not possible.要彌補(bǔ)失去的時(shí)光是不可能的。
To lean out of the window is dangerous.身子探出窗外很危險(xiǎn)。
To save money now seems impossible.現(xiàn)在好像不可能存錢。
(2)用it作形式主語，把真正的主語不定式置于句后，常用于下列句式中。如：
①It+be+名詞+to do
It's our duty to take good care of the old.照顧老人是我們的責(zé)任。
It is an offence to drop litter in the street.在馬路上亂丟廢物是違章的。
②It takes sb+some time+to do
How long did it take you to finish the work?你花了多少時(shí)間來完成這項(xiàng)工作?
③It+be+形容詞+for sb+to do
It is difficult for us to finish writing the composition in a quarter of an hour.我們難以在四分之一小時(shí)內(nèi)完成這篇作文。
It is stupid of you to write down everything (that) the teacher says.你把老師說的所有東西都記下來的行為是很愚蠢的。
④It seems(appears)+形容詞+to do
It seemed impossible to save money.
在句型③中，常用表示客觀情況的形容詞，如：difficult,easy,hard,important,impossible,necessary 等;在句型④中，常用careless,clever,good,foolish,honest,kind,lazy,nice,right,silly,stupid,wise等 表示贊揚(yáng)或批評(píng)的詞。在不定式前的sb，可看作其邏輯主語。這一句式有時(shí)相當(dāng)于Sb is+形容詞+to do句式 ，如：It's kind of you to help me with my English.=You are kind to help me with my English.
⑤It + 不定式結(jié)構(gòu)可位于believe/consider/discover/expect/find/think和wonder之后
He thought it would be safer to go by train.他認(rèn)為乘火車比較安全。
He will find it is hard to make friends.他會(huì)感覺到交朋友困難。
⑥不定式的完成式也可作句子的主語
To have made the same mistake twice was unforgivable.兩次犯同樣的錯(cuò)誤是不可原諒的。
It is better to have loved and lost than never to have loved at all.愛過而后失去了愛，比從未愛過要好。
(3)舉例
(1) It's easy (for me) to do that.我做這事太容易了
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.聽到你的聲音真高興。
It's necessary for you to lock the car when you do not use it.當(dāng)你不用車的時(shí)候，鎖車是有必要的。
(2) It's very kind of him to help us. 他幫助我們，他真好。
Kind, nice, stupid, rude, clever, foolish, thoughtful, thoughtless, brave, considerate(考慮周到的), silly, selfish(自私的)
例句:
It was silly of us to believe him. 我們真愚蠢，竟然相信了他。
It seemed selfish of him not to give them anything. 他不給他們?nèi)魏螙|西，這顯得太自私了。
注意：(1) 其他系動(dòng)詞如，look，appear等也可用于此句型
(2) 不定式作為句子成分時(shí)，動(dòng)詞用單數(shù)形式。
(3) 當(dāng)不定式作主語的句子中又有一個(gè)不定式作表語時(shí)，不能用It is… to…的句型
(對(duì))To see is to believe. 眼見為實(shí)。
(錯(cuò))It is to believe to see.
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