1．在助动词或情态动词之后，如be going to, used to, have to, ought to, be able to, be about to等。

　 She must go but you don't have to.　 她必须走，但你没有必要。

　 ---Do you think I ought to go to see my doctor　 你认为我应该去看医生吗？

　 ---Yes, I think you ought to.　 是的，我想你应该去。

8．在感官动词see, hear, watch, feel, notice以及使役动词make, let, have等后作宾语补足语的动词不定式不带to。

　 She watched the children cross the street. 她看着孩子们穿过了马路。

　 They made the boy apologize to his friend for being so rude. 他们让那男孩因他的粗鲁而向他的朋友道歉。

　 Don't forget to have him come earlier. 别忘了让他早点儿来。

　 B．不定式符号的单独使用

　 为了避免重复上文中出现过的动词，可以用不定式符号to来代替上文中出现过的不定式结构。

7．在一些固定搭配中用不带to的不定式。

　 He let go the rope. 他松开了绳子。

　 I hear say there will be an earthquake soon. 我听说不久就要有一次地震。

　 She made believe she was innocent. 她假装清白。

6．在help之后，既可用带to的不定式也可用不带to的不定式。

　 Can I help (to) carry the box for you 我帮你搬箱子，好吗？

5．在两个动词不定式并列使用时，为了避免重复，后面的不定式符号to可省略。

　 The little girl hardly knew whether to laugh or cry. 这小姑娘不知道是哭好还是笑好。

　 Can you help me to call him and ask him to attend the meeting at 200 p.m. 你能不能帮我打电话给他，叫他下午两点来开会？

4．如不定式前有行为动词do，那么在表语从句中的不定式和介词except或but之后的不定式可不带to。

　 The only thing I could do was go home. 我能做的唯一一件事就是回家。

　 They could do nothing but wait for the doctor to come. 他们只能等待医生的到来。

3．在had better, had best, would rather, would rather...than, would sooner, would sooner...than, cannot but, cannot choose but, cannot help but等结构后直接跟动词原形或not +动词原形。

　 You'd better listen to your teacher's opinion.　 你最好听一听老师的意见。

　 We had best call for the doctor at once. 我们最好马上就请医生来。

　 She can't do anything but ask silly questions. 她一直在问一些愚蠢的问题。

　 Rather than ride on a crowded bus, he always prefers to ride a bicycle. 他宁愿骑自行车，也不愿挤公共汽车。

　 He cannot but move to another street, because his building has to be pulled down soon. 他不得不搬到另一条街上去，因为他住的大楼很快就要被推倒。

　 We cannot help but admire his courage.　 我们情不自禁地钦佩他的勇气。

2．在why引起的一些问句中，疑问词直接跟动词原形或not+动词原形。

　 Why make so much noise　 为什么发出这么大的噪音？

　 Why not join us　 为什么不加入我们？

　 A．不带to的不定式

1．在口语中，动词原形come和 go后可接不带to的不定式。

　 Go tell her. 去告诉她吧。

　 Come have a glass. 来喝一杯。

3．There be结构中，当说话人考虑必须有人去完成某件事时。

　 There is nothing to worry about. 没什么可担心的。

　 There is nothing to fear. 没有什么可害怕的。

　 比较：

　 There is a lot of work to do. 有许多事情要做。(表示某人必须做这工作)

　 There is a lot of work to be done. 有许多事情要做。(强调有许多事情必须做)

　 She has two letters to type. 她有两封信要打。(自己打)

　 She has two letters to be typed. 她有两封信要打。(别人打)

　 I．表示虚拟语气的不定式完成式。

　 I hoped to have met him at the railway station, but he didn't turn up. 我本希望在火车站接到他，但他未露面。

　 We would love to have gone to the match, but the tickets were all sold out. 我们原想去看这场比赛的，但票已售完了。

　 The plane was to have taken off at 9, but something went wrong. 飞机原计划九点起飞，但出现了一些故障。　　　　

　 J．用于感叹句。

　 不定式可用于感叹句，含有惊异、不满、惋惜或赞美等感情色彩。

　 To think that he should do this! 想想，他竟然干出这等事！(表示惊讶)

　 Oh, God, to see her dance ! 哦，天哪，看看她的舞蹈！(表示赞美)

　 To think that all the money has been wasted. 想想，所有的钱都被浪费掉了！(表示不满)