(1)把不定式置于句首。如： To get there by bike will take us half an hour.
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.
Learning without practice is no good.
动名词作主语时，也常用It句式。如： ①It's +no good(no use,fun,a pleasure,a waste of time)+doing…
It's no good reading in dim light.
It's no use sitting here waiting.
It's dangerous swimming in the sea in windy days.
这样用的形容词有expensive,nice,tiring等，但important,necessary则不适用于这种结构，应用不定式 代替，如：It'simportant for you to keep fit.
③There is no+doing
There is no saying what will happen next.
在这一结构中，动名词后常带宾语，相当于"It's impossible to…"结构。
It's no good eating too much fat.
It's no good for you to eat so much fat.
It's no use your pretending that you didn't know the rules. 二、作宾语
I decided to ask for my money back.
I decided that I would ask for my money back.
When our visit to the farm was over,we expected to startback on foot.
When our visit to the farm was over, we expected that wewould start back on foot.
②当复合宾语中的宾语是不定式时，先用形式宾语it代替不定式，把不定式置于补语之后，即：主语＋动 词＋it＋补语＋to do句式。如：
We think it quite important for us to learn a foreignlanguage well.
He feels it his duty to help the poor.
The enemy soldiers had no choice but to give in.
On Sunday afternoon I had nothing to do but watch TV. ⒉动名词作宾语
①以下动词后，只能接动名词作宾语，如：admit,appreciate,consider,delay,enjoy,finish,keep,imag ine,mind,miss,practise,resist,risk,save,suggest,don't mind,give up,insist,on, put off等。如：
I suggest spending our summer vacation in a seaside town.
You must give up smoking, for it does too much harm toyour health.
I should go to attend the birthday celebration instead of staying at home.
What about inviting Li Jun to make a speech?
动名词前的介词有时可以省略，如：have difficulty(in)doing,have no trouble(in)doing,lose no ti me(in)doing,prevent/stop…(from)doing,there is no use(in)doing等。
在need,require,want后接-ing形式，表示被动意义，也可接不定式，但要用被动形式，如：Your handwr iting needs improving(tobe improved). hate,love,like接不定式表示特定的未来事件，接动名词表示目前 正在进行的活动或一般的行为。
①hate,like,love前有would(should)时，如：I'd like to have a cup of coffee.
②当谓语动词begin,continue,start等是进行式时，如：Thestudents are starting to work on the di fficult maths problem.
③begin,continue,start与know,understand等状态动词连用时，如：I soon began to understand what
Our teachers don't permit our swimming in the lake.
Our teachers don't permit us to swim in the lake.
Don't forget fo post the letter for me.
Have you forgotten meeting her in Beijing Airport?
Remember to close the windows before you leave.
I remember writing him a letter a year ago.
We regret to tell you that all of you are not invited toattend the meeting.
They regretted ordering these books from abroad. ②mean to do 打算做某事 doing 意味着……
I meant to catch up with the early bus.
This means wasting a lot of money. ③try to do 设法尽力做某事 doing 试着做某事
You should try to overcome your shortcomings.
Try working out the physics problem in another way. ④stop to do 停下一件事去做另一件事（不定式作目的状语） doing 停止做某事 On the way to the airport,I stopped to buy a paper.
You'd better stop arguing and do as you are told.
⑤can't help doing 禁不住…… to do不能帮助干……
They couldn't help jumping up at the news.
Sorry I have lots of work to do.So I can't help to make up the room for you.
⑥go on to do 做不同的事或不同内容的事 doing 继续不停地做某事，指同一动作的继续
He went on to talk about world situation.他接着又谈了世界形势。
We'll go on fighting so long as there is oppression inthe world.
⑦leave off to do 离开某地去干什么（目的状语） doing停下某事
It's time to leave off talking and to start acting.
They left off to go fishing. 三、做表语