什么时候使用动词不定式?

什么时候使用动词不定式啊?(简要回答)

动词不定式由to+动词原形构成。这里的to是不定式标志,没有词义。不定式具有名词、形容词或副词的某些语法功能,又有动词的时态和语态的特点及作用。
有些动词可以用不定式作宾语,大致上有三种情况。
1)有些及物动词用不定式作宾语,结构为动词+不定式。例如:
afford,aim,agree,arrange,ask,decide
例如:
The driver failed to see the other car in time. 司机没能及时看见另一辆车。
He offered to help me. 他表示愿意帮助我。
2)有些动词除了可以用不定式作宾语,还用不定式作补语,即有动词+宾语+不定式的结构。例如:
ask,choose,expect,help,beg,intend
例如:
I like to keep everything tidy. 我喜欢每件东西都保持整洁。
I like you to keep everything tidy. 我喜欢你使每件东西都保持整洁。
I want to speak to Tom.  我想和汤姆谈话。
I want you to speak to Tom.  我想让你和汤姆谈话。  
3)有些动词或动词词组可以用动词+疑问词+不定式的结构作宾语。例如:
decide, know,consider,forget,learn,remember
例如:
Please show us how to do that. 请演示给我们如何去做。
There are so many kinds of tape-recorders on sale that I can't make up my mind which to buy.
有这么多的录音机,我都拿不定主意买哪一种。

省去to 的动词不定式
1) 情态动词 ( 除ought 外) 后。
2) 使役动词 let, have, make后,感官动词 see, watch, look at, notice , observe, hear, listen to, smell, feel, find 等后。
注意:被动语态中不能省去to。例如:
I saw him dance. 我看见他跳舞。
=He was seen to dance.
The boss made them work the whole night. 老板让他们整夜干活。
=They were made to work the whole night.
3) would rather,had better句型后
4) Why… / why no…句型后
5) help 后可带to,也可不带to, help sb (to) do sth:
6) but和except后。but前是实义动词do时,后面出现的不定式不带to。
比较:He wants to do nothing but go out. 他只想出去玩。
He wants to believe anything but to take the medicine. 除了吃这药,他什么都信。
7) 由and, or和than连接的两个不定式,第二个to 可以省去:
8) 通常在discover, imagine, suppose, think等词后作宾补时,可以省去to be。例如: 
He is supposed (to be) nice. 他应该是个好人。
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