2、不定式的句法功能：
1）作主语：
To finish the work in ten minutes is very hard.
To lose your heart means failure.
动词不定式短语作主语时，常用形式主语it作形式主语，例如上面两句可用如下形式：
It is very hard to finish the work in ten minutes.
It means failure to lose your heart.
2）作表语：
Her job is to clean the hall.
He appears to have caught a cold.
3）作宾语：
常与不定式做宾语连用的动词有：want, hope, wish, offer, fail, plan, learn, pretend, refuse, manage, help, agree, promise, prefer, 如果不定式（宾语）后面有宾语补足语，则用it作形式宾语，真正的宾语（不定式）后置，放在宾语补足语后面，例如：
Marx found it important to study the situation in Russia.
动词不定式也可充当介词宾语，如：
I have no choice but to stay here.
He did nothing last Sunday but repair his bike.
动词不定式前有时可与疑问词连用，如：
He gave us some advice on how to learn English.
4）作宾语补足语：
在复合宾语中，动词不定式可充当宾语补足语，如下动词常跟这种复合宾语: want, wish, ask, tell, order, beg, permit, help, advise, persuade, allow, prepare, cause, force, call on, wait for, invite.
此外，介词有时也与这种复合宾语连用，如：
With a lot of work to do, he didn’t go to the cinema.
有些动词如make, let, see, watch, hear, feel, have等与不带有to的不定式连用，但改为被动语态时，不定式要加to, 如：
I saw him cross the road.
He was seen to cross the road.
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