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 哟! 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。)
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