不定式基本形式：to+动词原形，其中有一部分不定式是没有to的，那么哪些情况下，不定式没有to呢？
1、感官动词这几个感官动词feel ,hear ,smell,look,在主动语态里后接动词原形(无to)。例如：I saw him sing. I heard you play the piano. He feels him fly. I watched the girl cross the road.
2、使役动词使役动词let、 have 、make后接不定式没有to.
例如：You must have your son see his own mistakes. Let me go. Don't make her laugh.
3、在why not 后没有to，基本结构可记忆why not do 更容易
(1)表示提建议
例如：
Why not go with him?Why not have a rest?
（2）表示认为没有必要例如：Why worry. you can't do anything about it ?Why waste?
4、had better do 最好做…和would rather do 宁可做…后面的不定式没有to. '
You'd better drink the water now. or you will not be fine. I would rather go by bus now
5、Will you please do 征求意见，求助。例如：Will you please open the window?Will you please give me a hand ?
6、help 可以有to 也可以没有toHelp the boy to carry the heavy box. Help the old man water the flowers.
7、can't … but …，do nothing …but…结构中没有to
例如：We can't choose but obey
He can't help but feel sorry
for her.
We do nothing but wait.
以上是常用的不定式中不带to的几种情况，但是如果在被动语态中还是有to的，例如：I saw her danceShe was seen to danceMake her laugh She was made to laugh 这些情况在被动语态里在详细讲解。
