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

不定式作定語(yǔ)表示動(dòng)賓（或介賓）關(guān)系，本質(zhì)上是定語(yǔ)從句的簡(jiǎn)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(yǔ)從句中劃線部分省略，就得到不定式作定語(yǔ)。由此也可以看出不定式作定語(yǔ)仍然是主動(dòng)意義，而不是主動(dòng)形式表示被動(dòng)意義。