介词+which+to do为带关系代词的不定式短语，这是正式语体形式。假如把介词移到不定式末尾，省略关系代词，就是普通的不定式了：to do +介词。

不定式作定语表示动宾（或介宾）关系，本质上是定语从句的简略形式。比如：

I have a letter to write

= I have a letter which I am to wirte.

I have something to talk about.

= I have something that I am to talk about.

I have something about which to talk.

= I have something about which I am to talk.

I have someone to discuss the matter with.

= I have someone who I am to discuss the matter with.

I have someone with whom I am to discuss the matter.

定语从句中划线部分省略，就得到不定式作定语。由此也可以看出不定式作定语仍然是主动意义，而不是主动形式表示被动意义。