(1) 使役动词 let，make，have 等和感官动词 see, hear 等后省略 to。用于被动语态时必须带 to。
His mother wouldn't let him go to the movie.
His workers were made to workfor twelve hours a day.
(2)在动词 help 后用作宾语或宾语补足语的不定式可以不带to。当 help 用于被动语态时，其后不定式中的 to 不能省略。
I'll help ( to ) do that for you.
We were helped by the teacher to do the work.
(3)作表语时，主语从句中有行为动词 do 时，不定式 to 可以省略。
What he did was ( to )give up the project.
What I do is( to ) want you to understand me.
(4)句中含有行为动词do，在 but，except，besides, such as 等的后面省略to。
Don't do anything silly, such as marry him.
He can do nothing, but go home.
(5)不定式后有 rather than，rather than 后省略 to。
I prefer to stay at homerather than go shopping.
(6) why not, had better,
would rather, can't but 等词组后面省略to。
Why not go around our school?
You had better stayat home.
Her job is to take care of the children
and wash clothes.