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同学你好~
常需要使用不定式的场合是：
　　
一、常接不定式作宾语的动词有want，love，learn，agree，decide，hope，refuse等。例如：
I'm learning to skate on real ice. 我正在真正的冰上学滑冰。
不定式短语作宾语时，如果还带有宾语补足语，通常用it作形式宾语，而把不定式短语放在宾语补足语之后。例如：
He found it very difficult to get to sleep.他发现入睡很困难。

二、动词不定式常用作目的状语。例如：
He came to give us a talk yesterday. 他昨天来给我们作了个报告。
Mrs Brown went to see her doctor. 布朗夫人去看她的医生了。

三、tell，ask，want，order，get，wish，warn，teach，invite等动词后常跟不定式作宾语的补足语，构成tell/ ask……sb to do sth(“告诉/请……某人做某事”)结构。例如：
Could you ask him to call me，please? 请你让他给我打个电话，好吗?
使役动词let，make，have等，感官动词see，hear，feel，watch，notice，look at，listen to等，后常跟不带to的不定式作宾语补足语。例如：
A woman saw it happen when she was walking past. 一位妇女路过时看到了。
We often heard her sing in the next room. 我们经常听到她在隔壁唱歌。
注意：在被动语态句子中，不定式前必须加to.例如：
He was made to do it. 他被迫做这件事。
She was often heard to sing in the next room. 经常听到她在隔壁唱歌。

四、不定式短语作定语要放在被修饰的词语之后。例如：
I have a lot of work to do. 我有许多事情要做。

五、不定式短语作主语时，常用it作形式主语，真正的主语不定式短语放在后面。例如：
It is right to give up smoking. 戒烟是有益的。

六、作表语。例如：
To him，the first important thing was to save lives. 对他来说，最重要的事情就是拯救生命。
His wish is to become a scientist. 他的愿望是当一名科学家。

七、动词不定式用在疑问词how，when，where，what，which等之后，作宾语或宾语补足语。例如：
He taught us how to use a computer. 他教我们如何使用计算机。
I don't know where to go? 我不知道去哪里。


“介词+关系代词+不定式”的表现形式通常为 in which to do sth, with which to do sth, on which to do sth等，其中的介词视具体语境的搭配而定。整个结构修饰前面的名词。在正式文体中，当引导定语从句的关系代词用作介词的宾语时，有时可用“介词+关系代词+不定式”这样的结构来简化定语从句。
She must have time in which she can grow calm.=She must have time in which to grow calm. 她必须有冷静下来时的时间。
Allow me one minute in which I can change my clothes.=Allow me one minute in which to change my clothes. 给我一点时间换衣服。
He was miserable unless he had neighbors with whom he could quarrel.=He was miserable unless he had neighbors with whom to quarrel. 他要是没有邻居吵架就难受。

“介词+关系代词+不定式”结构中的介词不能没有，并且该介词也不能位于不定式后面。如：
在那儿孩子们有个玩耍的花园。
正：There the children had a garden in which to play. (很正式)
正：There the children had a garden in which they could play. (较正式)
正：There the children had a garden to play in. (较口语化)
误：There the children had a garden which to play in.

祝同学学习进步~