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 no…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。
典型例题
1） ---- I usually go there by train.
---- Why not ___ by boat for a change?
A. to try going B. trying to go C. to try and go D. try going
答案：D. why not 后面接不带to 的不定式，因此选D。
2） Paul doesn't have to be made ___. He always works hard.
A. learn B. to learn C. learned D. learning
答案：B. make后接不带to 的动词不定式，当其用于被动时，to 不可省略。