一、用法实例
“介词+关系代词+不定式”的表现形式通常为 in which to do sth, with which/whom to do sth, on which to do sth等，如：
They professor can hardly find sufficient grounds on which to base his argument in favour of the theory.
那位教授几乎无法找到充分理由来据以建立支持其新理论的论点。（考研真题）
She is a nice person with whom to work.
她是一个容易共事的人。
She must have time in which to grow calm. 她需要有时间冷静下来。
She had no time left in which to pack her things. 她没有时间收拾行李了。
He had only the long nights in which to study. 他只有漫漫长夜可用来学习。
Allow me one minute in which to change my costumes. 给我一点时间来卸装。
Now he had no pretext on which to prolong his stay. 现在他已没有继续留下去借口。
He had a couple of revolvers with which to defend himself. 他只有一两把手枪用来自卫。
She had some money in the bank, with which to help her mother. 她银行里有些钱用来帮助她的母亲。
The boy wants another fifty cents with which to buy some ice cream. 这男孩还要五角钱去买冰淇淋。
It was a marvelous place to begin one’s management career. First of all, there were so many different aspects of transport on which to try my hand.
这是开始一个人的经营管理生涯的好地方。首先，那里有涉及运输许多方面工作让我初试身手。
二、结构分析
在阅读中我们经常可以见到以上这样的“介词+关系代词+不定式”形式，许多读者不明白它是什么结构，其实它就是一种紧缩了的定语从句。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。比较：
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。
She had some money in the bank, with which she could help her mother. =She had some money in the bank, with which to help her mother. 她银行里有些钱用来帮助她的母亲。
三、补充说明
对于“介词+关系代词+不定式”结构，若语义清楚，且结构恰当，有时可进一步省略其中的关系代词，而只保留介词和不定式（注意，此时介词要移至不定式之后，且当先行词为表时间的名词时，该介词通常省略）。注意，这一结构中的关系代词通常只有which（指物）和whom（指人）如：
She must have time to grow calm (in). 她必须有冷静下来时的时间。
Allow me one minute to change my clothes (in). 给我一点时间换衣服。
He was miserable unless he had neighbors to quarrel with. 他要是没有邻居吵架就难受。
It’s reported that the city needs more land to build houses on. 据报道，这座城市需要更多的土地来建房。
Mary needs a friend with whom to play.
玛丽需要一个朋友和她玩耍。
“介词+关系代词+不定式”结构和“介词+疑问词+不定式”结构的差异：疑问词不限于which和whom,其他疑问词也是可以的，如：
The knowledge of how to do it was not common.
知道如何做这件事并非人人都能做到。
The question of what to sell is still undecided.
该卖些什么这个问题仍未确定。
“介词+疑问词+不定式”结构在句中具有名词性功能，相当于名词性从句。上二例可以改写为：
The knowledge of how it should be done was not common.
The question of what we should sell is still undecided.
而“介词+关系代词+不定式”结构中的关系代词从根源上理解必然是指代前面的先行词的，“介词+疑问词+不定式”结构中的疑问代词则没有这种指代功能。试比较：
He is the best man with whom to discuss the matter.
他是商量这件事的最佳人选。（句中whom是关系代词，指代前面的先行词the best man,全句相当于He is the best man with whom we can discuss the matter.）
We talked about the question of whom to elect.
我们讨论了选举谁（人选）的问题。（句中whom是疑问代词，全句相当于We talked about the question of whom we should elect.）
本人独家研究发现：在“介词+关系代词+不定式”结构中的介词是隶属于其后的不定式的（所以才有该结构中介词的选择，主要根据不定式动词的搭配以及不定式所修饰的名词的需要来决定的）；在“介词+疑问词+不定式”结构中介词是隶属于前面的所修饰词的。可以说用这种理论解释它们的区别是一锤定音的。
在“介词+关系代词+不定式”结构中的不定式有两种形式：“介词+关系代词+不及物动词不定式”和“介词+关系代词+及物动词不定式+宾语”；请看例句：
I haven’t got a chair on which to sit.
我没有椅子坐。（on which +to sit(不及物)）
Here is the meter with which to test the device.
这就是用来检测设备的仪表。（with which+to test(及物)+the device(宾语)）
在“介词+关系代词+不定式”结构中的不定式只能是主动形式的不定式。所以下列的句子就不可转换成这种结构：
The government agrees to install services which are to be paid for be residents over a ten-year period.
政府同意提供服务设施，经费有居民在年期间偿还。
四、特别提示
值得指出的是，“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.