一、作主语，可以用it代替，it叫形式主语，动词不定式放在后面叫真正主语
1．To learn a foreign language is not easy.
It is not easy to learn a foreign language.
2. It’s dangerous to drive very fast.
二、作表语
My idea is to ring him up at once. 我的想法是马上给他打电话。
如果主语部分含实义动词do，且作表语的动词又是“do”的内容，这时表语不定式的“to”可以省略。如：All I did was wait here. 我能做的就是在这里等。
三、常可接动词不定式作宾语的动词有：agree(同意), hope(希望), decide(决定), need(需要), mean(打算), wish(希望), fail(失败), want(想要), begin(开始), would like(想要)等。
He has decided to go to the countryside. 他已决心去乡下。
四、“疑问词 + 动词不定式”结构可以作动词know、think、find out等的宾语。
I don’t know who to ask. 我真不知道该问谁。
五、“疑问词 + 动词不定式”可以作动词show、teach、ask等后面的直接宾语。
She told me where to find the earphone. 她告诉我什么地方可以找到耳机。
六、如果宾语太长，可用it作形式宾语，构成“主语 + 谓语 + it + 宾语不足语(名词或形容词) + to do sth”结构，谓语动词常为find、think、feel等。
I found it difficult to stop him. 我发现拦截他很困难。
七、动词不定式可以作介词的宾语。
Autumn harvest is about to start. 秋收即将开始。
I’m worrying about what to do next. 我正愁下一步该怎么办。
八、作定语的动词不定式与被修饰词有动宾关系。
I have a lot of books to read. 我有许多书要读。
此时，如果动词不定式动词为不及物动词，后面的介词绝对不能省略。
We had only a cold room to live in. 我们只有寒室一间。
九、作定语的动词不定式与被修饰词有主谓关系。
He is always the first to come and the last to leave. 他总是第一个到，最后一个走。
十、作定语的动词不定式与被修饰词之间只有修饰关系。
We have no time to go to town today. 今天我们没有时间去城里。
十一、作状语的动词不定式常表示目的、原因、方向、结果等。
I’m glad to meet you. 见到你，我很高兴。
They ran over to welcome the foreign guests. 他们跑过来欢迎外宾。
十二、不定式复合结构“for sb to do sth”可用作主语、表语、定语、状语等。
It’s for her to decide. 这得由她来决定。（表语）
There are many books for you to read. 这有许多书供你阅读。（定语）
The book ids too difficult for children to read. 这本书太难了，孩子们看不懂。（结果状语）
不定式复合结构作主语时，需注意：
It’s + 形容词 + for / of + sb + to do sth.
当上面的形容词指的是to do sth的性质时，用介词for。
It’s dangerous for children to swim in the river. 孩子在这条河里游泳很危险。（游泳这件事情危险）
当上面的形容词指的是sb的性质时，用介词of。这些形容词往往修饰人：good, bad, polite, unkind, kind, nice, clever, right, wrong, careful, careless
It was careless of you to do that. 你这么做真粗心。（你这个人粗心）
十三、动词hear, see, feel, watch, notice, look at, listen to等后面的动词不定式作宾语补足语时不能带to, 即常见的形式为: hear sb do sth等
Many people like to watch others play games. 许多人喜欢看别人玩游戏.
十四、let, make, have后面的动词不定式作宾语补足语, 也不能带to; help后面的动词不定式作宾语补足语, to可有可无
She let us meet her at the station, but she didn’t come. 她让我们去车站迎接她,卡她没有来.
十五、十三和十四这两种情况下的动词不定式在改被动语态句子时,必须将省略的to还原,也就是说,动词后需跟带to的动词不定式.
We heard him sing every day.
He was heard to sing every day. 那时每天都听到他唱歌.
十六、跟带to的动词不定式作宾语补足语的动词还有: ask, beg, leave, like, love, hate, prefer, order, teach, tell, believe, find, know, want, think, understand, would like等
形式为: ask sb to do sth
Would you like me to visit him? 要不要我拜访他?
I would prefer you not to come tomorrow. 我宁愿你明天不要来.
He ask the driver to stop the motobike. 他要那位司机拦下那辆摩托车.
十七、区别下列词组的不同含义:
①like to do sth
like doing sth
②stop to do sth
stop doing sth
③remember to do sth
remember doing sth
④forget to do sth
forget doing sth
十八、It takes sb some time to do sth.花费某人一段时间干某事.
It took me half an hour to do my homework yesterday.昨天我花了半小时做回家作业.
It took Jim an hour and a half to read the book.吉姆花了一个半小时看这本书.
It took mother one and a half hours to do the housework.母亲花了一个半小时干家务.
用汉字表述上面的句型为:It + take + 人 + 一段时间 + to do sth.
一段时间形式往往为像这样的例子:fifteen minutes15分钟,an hour一个小时, two days两天, half a month半个月,a year and a half一年半...等等
十九、不定式的特殊句型too…to…
①too…to 太…以至于…
He is too excited to speak.
他太激动了，说不出话来。
---- Can I help you ? 需要我帮忙吗?
---- Well, I'm afraid the box is too heavy for you to carry it, but thank you all the same.不用了。这箱子太重，恐怕你搬不动。谢谢您。
②如在too前有否定词，则整个句子用否定词表达肯定， too 后那个词表达一种委婉含义，意 为"不太"。
It's never too late to mend.(谚语)
改过不嫌晚。
③当too 前面有only, all, but时，意思是：非常… 等于very。
I'm only too pleased to be able to help you. 我非常高兴能帮助你。
He was but too eager to get home. 他非常想回家。
二十、不定式的特殊句型Why not
"Why not +动词原形"表达向某人提出建议，翻译为："为什么不……?" "干吗不……?"
例如：
Why not take a holiday?
干吗不去度假?
练习
根据括号内的动词,用适当的形式填空,空内可能不只填一个词,使句子完整意思正确
1. They usually __________ kites on Sundays. (fly)
2. Look! The dogs __________ after the cat. (run)
3. Miss Gao __________ English in our school since she came here. (teach)
4. The class meeting _________ last Wednesday afternoon. (hold)
5. You should let him __________ the work by himself. (finish)
6. Meimei __________ to wash the old woman’s clothes twice every week. (go)
7. Mr Jiang __________ us Chinese last term. (teach)
8. My father __________ back from Japan tomorrow. (come)
9. It’s seven o’clock now. The Greens __________ breakfast together. (have)
10. Jim __________ many Chinese songs since he came to China. (learn)
11. Please remember __________ the window when you leave the room. (close)
12. After school some students began to do some __________ in the classroom. (clean)
13. English __________ widely in the whole world today. (speak)
答案
1. fly, 2. are running, 3. has taught, 4. was held, 5. finish, 6. goes / went,
7. taught, 8. is going to come / is coming / will come / comes 9. are having,
10. has learned, 11. to close, 12. cleaning,
13. is spoken
