你这问题有个前提：就是不定式作后置定语.
He has a lot of work to do.
to do 的逻辑宾语是a lot of work ,即动词不定式与它所修饰的词之间有动宾关系.
to do 的动作由主语来执行,也就是说它的逻辑主语是主语,不定式是主语的逻辑谓语,也就是说：to do 与主语是逻辑上的主谓关系.
另：如果动词不定式与句子的主语没有主谓关系,要用不定式的被动形式.如：
Tom,I am going to shopping ,Do you have anythin to be bought?
句子主语是you ,而不定式的执行者是I,没有主谓关系,用了to be bought被动形式.