不定式省to有四种情况：
使役动词 let,have,make 等后接不定式。如：Let him go! 让他走!
would rather，had better后。如：You had better stay at home.你最好呆在家里。
Why... / why not...后。如：Why not have a good rest on Sunday? 为什么星期天不好好休息一下呢?
感官动词 see, watch, look at, hear, listen to, smell,feel, find 等后作宾补，省to。如：I saw him dance. 我看见他跳舞。
注意：这些情况在被动句中可千万不可省to ！如：The boss made them work the whole night. 变成被动句：They were made to work the whole night.
不定式的特殊用法：
It与不定式：动词不定式可以做主语，但如果动词不定式太长，显得头重脚轻的，那么我们就可用形式主语it代替，而把真正主语(即不定式)放于句尾。
如: It is not difficult for me to study English well.(对我来说学好英语是可能的。)
不定式还可以充当句子的宾语，但有些动词，如find，think，believe 等，在语法上不能接受不定式作宾语，只有用it作形式宾语，从而把动词不定式置于句尾。这样的不定式可继续充当其宾语的作用。
如: I found it difficult to fall asleep.我发现很难入睡。
还有一点：动词不定式，还可用在how，when，where，what，which 等疑问代词或副词之后，与其共同作宾语。如：
I don't know how to use a computer.我不知道怎样使用电脑。
Ask me if you're not sure how to do it.
你要是拿不准怎么干，就问我。
She's still learning how to dance.
她仍在学怎样跳舞。
I don't know how to make polite conversation.
我不晓得怎么说应酬话。
I'll show you how to load the software.
我给你演示一下如何装入这套软件。
You have to know how to handle yourself in this business.
你必须知道在这件事上如何把握自己。
Today we learnt how to use the new software.
今天我们学习了怎样使用这个新软件。
Do you know how to work the coffee machine?
你会使用咖啡机吗？
We discussed everything ─ when to go, what to see and so on.
我们什么都商量过了——什么时候走、看什么等等。
I will contact you later to advise you when to come.
稍后我会与你联系，通知你何时前来。
The problem lies in deciding when to intervene.
问题在于决定何时介入。
He told us when to indicate and when to change gear.
他告诉我们何时打转向灯、何时换挡。
You have to know what's what and when to draw the line.
你必须搞清状况以及自己的底线。
He knew when to leave well alone and when to interfere.
他知道什么时候该适可而止，什么时候该插手干预。
Like many politicians before him, Mr Bentley did not know when to stop
和过去的很多政客一样，本特利先生做事也没分寸。
He pondered on when to give her the ring.
他斟酌着何时给她戒指。
A man among men is he who knows when to eat humble pie and when to hold his head high.
大丈夫能屈能伸。
He was considering what to do next.
他在考虑下一步怎么办。
I don't quite know what to do next.
我不太清楚下一步该怎么办。
She never had the least idea what to do about it.
这事怎么办，她一点主意都没有。
I don't know what to make of the new manager.
这位新经理，我不知道怎么评价他才好。
I just don't know what to do for the best.
我就是弄不清做什么才好。
I don't know what to do with all the food that's left over.
我不知道怎样处理所有这些剩饭剩菜。
I never know what to expect from one day to the next.
一天又一天我不知道该期待什么。
Don't tell me what to do!
别对我指手画脚的！
I can't decide what to wear.
我拿不定主意穿什么。
He was unsure of what to do next.
他对下一步该做什么犹豫不定。
We rarely agree on what to do.
我们很少在要做的事情上看法一致。
They believed the witnesses had been coached on what to say.
他们认为证人所作的证词是别人教他们那样说的。
The officer briefed her on what to expect.
长官简要向她说了一下可能遇到的情况。