It is the key to open the box with.
with在里面起到什么作用,为什么后置?
如果作定语的动词不定式(短语)所修饰的名词是该不定式动作的工具,应该在动词不定式短语后面使用适当的介词。如:I have no pencil to write with. (= I have no pencil with which to write.)我没有写字的铅笔。