介词+which/whom+to do不定式，称为带关系代词的不定式。是一种定语从句的简略形式，通常用于正式书面语体。这种不定式只能作定语。你可以理解这里省略了主语+be，也可以理解为省略了主语和情态动词，情态动词后的不定式改为to do不定式。例如下句：

I need a knife with which I can cut the watermelon. 我需要一把切西瓜的刀。

把这个句子中的 I can cut 改为to cut, 定语从句就简略为一个不定式短语作定语了。I need a knife with which to cut the watermelon.

非正式语体的定语从句由which或that引导，关系代词还可以省略，介词with留在句末。

I need a knife (that/which) I can cut the watermelon with.

这个定语从句也可以简略为不定式作定语，即省略关系代词，把 I can cut改为to cut, 得到：

I need a knife to cut the watermelon with.

你现在比较下面二个句子：

This is a comfortable chair on which to sit. 正式语体。

= This is a comfortable chair on which one can sit.

This is a comfortable chair to sit on. 非正式语体。

= This is a comfortable chair (that) one can sit on.