众所周知，不定式由“to+动词原形”构成，这里的的to是不定式符号，没有词义，而且有时可以省略。现将不定式符号to省略的几种情况归纳如下:
1，当动词不定式在情态动词can,must, need, dare之后时。例如:
He can repair the computer.他会修电脑。
You must pay the money, but you needn't do so at once.这些钱你必须付，但不必马上付。
2. 当动词不定式在半助动词had better 和would rather之后时。例如:
You'd better ask that policeman over there.你最好去问那边那个警察。
The boy would rather drink cold water.那男孩宁愿喝冷水。
3.在“why(not)+动词不定式”结构中。例如:
Why not play football?干嘛不踢足球呢?
Why spend so much money on books?为什么花这么多钱买书?
4.在“will (would) you+不定式”句型中。例如:
Will you please help me?请帮帮我，好吗?
Would you please tell me the way to the post office?你们能告诉我去邮局的路怎么走吗?
5. 两个或两个以上的动词不定式连用时，如果它们在句中的语法功能相同，通常只需第一个动词不定式带to，其余的都不带to。例如:
Lucy taught her brother to read and write.露西教她弟弟读书写字。
Miss Gao asked me to go and get some chalk for her.高老师叫我去给她拿粉笔。
6.表示生理感觉的动词(如 see,hear, watch, feel 等)后面跟不带的不定式作宾语补足语。例如:
I'm watching the boys swim.我在看这些孩子游泳。
I often hear her sing this song.我经常听见她唱这首歌。
7.一些使役动词(如 let,make,have等)后面跟不带的不定式作宾语补足语。例如:
The teacher had the boy stand with his back to the blackboard.老师叫那孩子背向黑板站着。
How did Ling Feng make the baby stop crying?凌风是怎样使婴儿停止啼哭的。
但是，当这些感觉动词和使役动词用在被动语态的句子中，其后又必须跟带to 的不定式作主语的补足语。例如:
Though he had often made his little sister cry, today he was made to cry by his little sister.虽然他经常弄得他小妹妹笑，可是今天他被他小妹妹弄哭了。