一、动词不定式作主语。动词不定式在句中作主语时,谓语动词用单数。有时为了保持句子的平衡,常用it作形式主语放在句首,而把真正的主语——动词不定式(短语)放在后面。

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(    )1. To do as many exercises as possible ______ necessary before exam.
A. are    B. is      C. have      D. has
(    )2. It’s a good habit ______ breakfast every day.
A. had    B. have    C. has       D. to have
二、动词不定式作宾语与v-ing形式作宾语的区别。有些动词之后只能用v-ing形式,如:enjoy, mind, practice, keep等;有些动词之后只能用动词不定式,如:afford, agree, decide, fail等;有些动词之后既可用v-ing形式又可用动词不定式,二者没有什么区别,如:continue, begin, learn, like等;有些动词之后用v-ing形式和动词不定式意义有所不同,如:try, forget, mean, remember等
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(    )3. —Oh, terrible! I forgot ______ the window. It’s windy.
—Really? Let’s go back home quickly.
A. closing B. to close    C. closed     D. close