1) 情态动词 ( 除ought 外) 后。
2) 使役动词 let, have, make后,感官动词 see, watch, look at, notice , observe, hear, listen to, smell, feel, find 等后。
注意:被动语态中不能省去to。例如:
I saw him dance. 我看见他跳舞。
=He was seen to dance.
The boss made them work the whole night. 老板让他们整夜干活。
=They were made to work the whole night.
3) would rather,had better句型后
4) Why… / why not…句型后
5) help 后可带to,也可不带to, help sb (to) do sth:
6) but和except后。but前是实义动词do时,后面出现的不定式不带to。
比较:He wants to do nothing but go out. 他只想出去玩。
He wants to believe anything but to take the medicine. 除了吃这药,他什么都信。
7) 由and, or和than连接的两个不定式,第二个to 可以省去:
8) 通常在discover, imagine, suppose, think等词后作宾补时,可以省去to be。例如:
He is supposed (to be) nice. 他应该是个好人。
参考资料:http://zhidao.baidu.com/question/41090229.html?si=6