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…：
6) help 可带to，也可不带to, help sb (to) do sth：
7) but和except：but前是动词do时，后面出现的动词用不带to的动词不定式。
8) 由and, or和than连接的两个不定式，第二个to 可以省去：
9) 通常在discover, imagine, suppose, think, understand等词后，可以省去to be：
He is supposed (to be) nice. 他应该是个好人。
