一、作主语
⒈不定式作主语
动词不定式作主语时，句子的谓语动词常用单数，其位置有以下两种：
(1)把不定式置于句首。如：
To get there by bike will take us half an hour.
(2)用it作形式主语，把真正的主语不定式置于句后，常用于下列句式中。如：
①It＋be＋名词＋to do
It's our duty to take good care of the old.
②It takes sb+some time+to do
How long did it take you to finish the work?
③It＋be＋形容词＋for sb＋to do
It is difficult for us to finish writing the compositionin a quarter of an hour.
④It＋be＋形容词＋of sb＋to do
It is stupid of you to write down everything the teachersays.
⑤It seems(appears)＋形容词＋to do
It seemed impossible to save money.
在句型③中，常用表示客观情况的形容词，如：difficult,easy,hard,important,impossible,necessary 等；在句型④中，常用careless,clever,good,foolish,honest,kind,lazy,nice,right,silly,stupid,wise等 表示赞扬或批评的词。在不定式前的sb，可看作其逻辑主语。这一句式有时相当于Sb is＋形容词＋to do句式 ，如：It'skind of you to help me with my English.=You are kind to helpme with my English.
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