同学你好，该知识点来自沪江网校《新概念英语第四册【天猫专享】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好~
有时可以见到“介词+关系代词+不定式”这样的结构（可视为一种紧缩的定语从句）。关系代词+不定式多用于介词的后面，比较：

She must have time in which to grow calm.
=She must have time in which she can grow calm. 她必须有冷静下来时的时间。

She had some money in the bank, with which to help her mother.
=She had some money in the bank, with which she could help her mother.
她银行里有些钱用来帮助她的母亲。

值得指出的是，这类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，比较：

There the children had a garden in which to play.（很正式）
There the children had a garden in which they could play.（较正式）
There the children had a garden to play in.（较口语化）在那儿孩子们有个可以玩耍的花园。
但是不能说：There the children had a garden which to play in.

祝同学学习进步~