学帮网since water covers much of the earth ,he knew we should keep the seas clean.为什么用knew
来源：学生作业学帮网 编辑：学帮网 时间：2024/05/19 10:52:32
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.