since water covers much of the earth ,he knew we should keep the seas clean.为什么用knew

since water covers much of the earth ,he knew we should keep the seas clean.为什么用knew
since water covers much of the earth ,he knew we should keep the seas clean.为什么用knew

since water covers much of the earth ,he knew we should keep the seas clean.为什么用knew
前半句是陈述常识,自然,不需要过去式；后半句是描述已发生的事情,故要用过去式.