作业帮since water covers much of the earth ,he knew we should keep the seas clean.为什么用knew
来源：学生作业帮助网 编辑：作业帮 时间：2024/05/14 19:51:20
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.