句型：It is / was + adj. / n. +真正的主语
常用此句型的形容词有：clear, true, strange, obvious, (im) possible, (um) fortunate, good, funny, wonderful, (un) usual, pleasant, easy等；常用此句型的名词有：a pity, a wonder, ones duty, a mistake, no use, no good, a waste等。
真正的主语有三种：
1）用for或of引起的短语作不定式的逻辑主语,如for sb. to do sth. 或of sb. to do sth.
Its important for a Chinese to master a foreign language.
Its impolite of you to spit on the ground.
2）在no use, no good, a waste of...useless等词后，常用v.-ing(短语)作真正的主语
Its no use trying it again.
3）由that或连接代（或副）词引导的从句作真正的主语
Its strange that you should all think like that.
Its a problem whether the meeting will be held tomorrow.
动词不定式的主要功能
（1）作主语：To see one time is better than to hear a hundred times.
注：不定式作主语时，常用it代替之作形式主语，而不定式移置句子后部，以保持句子的平衡。不定式（短语）用作主语时，其后的谓语的动词要用单数。
e.g. It is a great honour for him to be invited to the big party.
（2）作表语：A nurses duty is to look after patients.
（3）作宾语：I dont know which one to buy.
Marx gave some advice on how to learn a foreign language.
（4）作定语：She is always the first to come and last to leave the classroom.
Give me some paper to write on, please.
注：不定式常作后置定语。不及物动词的不定式作定语，要加上适当的介词。
（5）作状语：The pupils ran over to welcome the foreign guests. (表目的)
What have I said to make you so sad? (表结果的)
The children jumped with joy to hear the exciting news. (表原因)
（6）作宾补：I wish you to be happy forever.
注：在watch, notice, see, look at, observe, hear, listen to, feel, let, make, have等感观和使役动词后的不定式复合结构中，不定式都不带to; 在help后既可带to,亦可省to.
e.g. In those days, the landlord made them work day and night.
这种句子如果变成被动语态，需将to补回。
e.g. In those days, the
