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。
