注意，作主语用的动词不定式常常为it替代放在句子开头，动词不定式(或短语)通常放在谓语后面，但翻译时可不必将it译出来，上面几个句子还可表述为：
① It is not easy to learn a foreign language.
② It is dangerous to play in the street.
③ It is important to grow plants.
如果要说明不定式表示的动作是谁做的，可以在不定式前加一个for引起的短语。
It's important for us to learn a foreign language.
对我们来说学好一门外语很重要。
It's a little painful to get an injection. 注射有一点疼。
It's not interesting to work in a hospital. 在医院工作不是很有趣。
五、同步练习
用括号内所给动词的适当形式填空。
1. I don't know when ______(have) a picnic.
2. She is very busy today. She has a lot of work ______(do).
3. It's impossible for us ______(finish) the work in such a short time.
4. teacher asked us ______(be) careful when we crossed the road.
5. Last week they went to the farm ______(help) the farmers with their work.
6. She wants ______(be) a doctor when she grows up.
7. How long does it take you ______(get) to the factory by bus?
8. My mother never lets me ______(play) football after lunch.
六、同步练习答案及讲解
1． to have（这是带有疑问词的动词不定式，在句中做宾语。）
2． to do（不定式做定语，修饰work。）
3． to finish（动词不定式做主语。）
4． to be（不定式做宾语补足语。）
5． to help（不定式做状语。）
6． to be（不定式做宾语。）
7． to get（不定式做主语。）
8． play（不定式做宾语补足语。有些动词如：let，make，feel，have，hear，see，watch等后面的动词不定式做宾语补足语时，动词不定式省掉to。）
《九年级Unit 15 A doctor for animals(第2页)》出自：www.89xue.com网