since water covers much of the earth ,he knew we should keep

问题描述:

since water covers much of the earth ,he knew we should keep the seas clean.为什么用knew
1个回答 分类:英语 2014-10-23

问题解答:

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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.
 
 
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