省to 的动词不定式

　　1） 情态动词 ( 除ought 外，ought to)：

　　2） 使役动词 let, have, make：

　　3） 感官动词 see, watch, look at, notice , observe, hear, listen to, smell, feel, find等后作宾补，省略to。
　　注意：在被动语态中则to 不能省掉。

　　　 I saw him dance.

　　　=He was seen to dance.

　　　 The boss made them work the whole night.

　　　=They were made to work the whole night.

　　4） would rather，had better：

　　5） Why… / why not…：