用所给动词的适当形式填空
Too much homework _________ (lead) to students’ poor health.
Work harder, and you               (get) better grades in your lessons.
John’s mother often hears him ____________（sing）in his room, and the songs are beautiful.
— Look! The little boy is swimming in the river.— I ________ (not know) he could swim.
Someone _____________ (knock) at the door, go and see who it is.
We should prevent people from                wild animals. (kill)
They did everything they could___________the animals in danger. (protect)
You will fail in the exam if you            (not be) careful.