在不定式短語(yǔ)作主語(yǔ)的句子中，往往用 it 作形式主語(yǔ)，而不把不定式短語(yǔ)放在句子的后面。如：
    To die like that is a terrible thing.=It is a terrible thing to die like that.
    象那樣死去是一件可怕的事。( it 作形式主語(yǔ)，把真正的主語(yǔ)to die like that放在后面)
    To know oneself is difficult.=It’s difficult to know oneself.
    有自知之明不容易。
    To tell a lie is not always easy.= It’s not always easy to tell a lie.
    撒謊并非總是容易。
    Never to offend anyone is my principle. 永遠(yuǎn)不得罪任何人就是我的原則。
    It’s against nature to remain single. 保持單身是違反自然的。
    To see is to believe.   眼見(jiàn)為實(shí)。
    For a dog to die like that is a terrible thing. = It is a terrible thing for a dog to die like that.
    一條狗象那樣死去是一件可怕的事。(for a dog是不定式的主語(yǔ))
    For one to know everything is impossible.= It’s impossible for one to know everything.
    一個(gè)人什么都知道是不可能的。
    For his brother John to go to sea seems natural.= It seems natural for his brother John to go to sea. 他兄弟John去當(dāng)水手是很自然的事。
    It’ll be a great shame for you to forget her. 你要是把她忘了，那可太不應(yīng)該了。
    It’s not proper for us not to accept the invitation. 我們不接受邀請(qǐng)不合適。(否定)
    It has not yet been decided when and where to discuss her resignation. 何時(shí)何地討論她的辭職還沒(méi)定。
    但是，下列形容詞做表語(yǔ)時(shí)，不定式的邏輯主語(yǔ)由 of 引出：
    careless粗心的，clever聰明的，cruel殘忍的，foolish愚蠢的，good好的，kind好心的 ，naughty淘氣的，nice好的，polite禮貌的，right正確的，rude無(wú)禮的 ，silly愚蠢的，stupid傻的，unwise不明智的，wise明智的，wrong錯(cuò)誤的，等等。
    為什么這些詞要用 of 引出不定式呢?
    因?yàn)檫@些形容詞都是描寫(xiě)人的性格和特點(diǎn)的形容詞?！癐t is + 形容詞 +of sb + to do sth”這樣的句型更強(qiáng)調(diào)某人如何如何，可以改寫(xiě)為：“Sb + be + 形容詞 + to do sth.” 而“for sb to do sth”這一結(jié)構(gòu)更強(qiáng)調(diào)做某事如何如何。例如：
    1、 It is difficult for me to learn English. 更強(qiáng)調(diào)學(xué)英語(yǔ)很難，不能寫(xiě)成
       I am difficult to learn English. (這句話(huà)含有邏輯錯(cuò)誤。)
    2、 It is unwise of you to go to the United States at this time. =You are unwise to go to the United States at this time. 強(qiáng)調(diào)你不明智。
    It’s very kind of you to come to see us off. =You’re kind to come to see us off. 你來(lái)為我們送行太好了。
    It’s unwise of him to drink and smoke so much.=He is unwise to drink and smoke so much. 他喝這么多酒、抽這么多煙是不明智的。
    It’s extremely cruel of John to kill animals. John殺害動(dòng)物真是殘忍之極。
    It’s rude/polite of Jack to say this to her.  Jack對(duì)她說(shuō)這個(gè)，很無(wú)禮/禮貌。