As the average age of the population increases, there are more and moreold people to care for.
问：此句中 to care for为不定式作old people的后置定语成动宾关系，为何不需要不定式的被动语态：to be cared for?
As the average ageof the population increases, there are more and more old people to care for.
句子暗示，不定式的逻辑主语是“大家；大家”。假如你在 to care for 之前添上省去的逻辑主语，就了解为何不需要被动语态了：
As the average ageof the population increases, there are more and more old people for us to care for.
尽管语言有抽象的语法规则进行规范，但它毕竟是很形象的东西，需要你认真去体验它，领会它的内在含义。比如有时，假如把不定式的主动形式改为被动形式，意思会发生非常大变化。比较：
There is nothingto do. 无事可做。（暗示“无聊感”）
There is nothingto be done.不可以干什么了。（暗示“束手无策”）
There is nothingto see. 没值得看的东西。
There is nothingto be seen. 什么东西也看不见。
I will findsomething to eat. 我要找点吃的。（自己吃）
I will findsomething to be eaten. 我要找点吃的。（自己或其他人吃）
He has a lot ofwork to do. 他有不少工作要做。（自己做）
He has a lot ofwork to be done. 他有不少工作要其他人去做。（他安排其他人做）
