1） 情態動詞 ( 除ought 外，ought to)：
2） 使役動詞 let, have, make：
3） 感官動詞 see, watch, look at, notice , observe, hear, listen to, smell, feel, find 等後作賓補，省略to。
注意：在被動語態中則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 可帶to，也可不帶to, help sb (to) do sth：
7） but和except：but前是動詞do時，後面出現的動詞用不帶to的動詞不定式。
8） 由and, or和than連線的兩個不定式，第二個to 可以省去：
9） 通常在discover, imagine, suppose, think, understand等詞後，可以省去to be：
he is supposed (to be) nice. 他應該是個好人。
舉例：
he wants to move to france and marry the girl.
he wants to do nothing but go out.
比較： he wants to do nothing but go out.
he wants to believe anything but to take the medicine.
典型例題
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 後面接不帶to 的不定式，因此選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後接不帶to 的動詞不定式，當其用於被動時，to 不可省略。