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 nurse's duty is to look after patients.
（3）作宾语：I don't 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, they were made to work day and night.
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