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。
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：
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。
He wants to move to France and marry the girl.
He wants to do nothing but go out.
比较： He wants to do nothing but go out.
He wants to believe anything but to take the medicine.
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。