介詞+which+to do為帶關系代詞的不定式短語，這是正式語體形式。假如把介詞移到不定式末尾，省略關系代詞，就是普通的不定式了：to do +介詞。

不定式作定語表示動賓（或介賓）關系，本質上是定語從句的簡略形式。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.

定語從句中劃線部分省略，就得到不定式作定語。由此也可以看出不定式作定語仍然是主動意義，而不是主動形式表示被動意義。