2.不定式的句法功能：

　　(1)作主語：

　　To finish the work in ten minutes is very hard.

　　To lose your heart means failure.

　　動詞不定式短語作主語時，常用it作形式主語，真正的主語不定式置于句后，例如上面兩句可用如下形式：

　　It is very hard to finish the work in ten minutes.

　　It means failure to lose your heart.

　　常用句式有：1、It+be+名詞+to do。2、It takes sb.+some time+to do。3、It+be+形容詞+of sb +to do。4、It+be+形容詞+for sb.+to do。常用careless,,clever,good,foolish,honest,kind,lazy,nice,right,silly,stupid,wise,等表示贊揚或批評的形容詞，不定式前的sb.可作其邏輯主語。

　　(2)作表語：

　　Her job is to clean the hall.

　　He appears to have caught a cold.

　　(3)作賓語：

　　常與不定式做賓語連用的動詞有：want, hope, wish, offer, fail, plan, learn, pretend, refuse, manage, help, agree, promise, prefer, 如果不定式(賓語)后面有賓語補足語，則用it作形式賓語，真正的賓語(不定式)后置，放在賓語補足語后面，例如：

　　Marx found it important to study the situation in Russia.

　　動詞不定式也可充當介詞賓語，如：

　　I have no choice but to stay here.

　　He did nothing last Sunday but repair his bike.

　　動詞不定式前有時可與疑問詞連用，如：

　　He gave us some advice on how to learn English.

　　(4)作賓語補足語：

　　在復合賓語中，動詞不定式可充當賓語補足語，如下動詞常跟這種復合賓語：want, wish, ask, tell, order, beg, permit, help, advise, persuade, allow, prepare, cause, force, call on, wait for, invite.

　　此外，介詞有時也與這種復合賓語連用，如：

　　With a lot of work to do, he didn't go to the cinema.

　　有些動詞如make, let, see, watch, hear, feel, have等與不帶有to的不定式連用，但改為被動語態時，不定式要加to, 如：

　　I saw him cross the road.

　　He was seen to cross the road.