在这个句子中,the light 是主语啊,主干是：the light don't come from the sun,but frome th earth.
而原句大概意思是,我们透过光线才看见月亮,而这光线来自地球而非太阳.（非正经翻译,甭较真）
看这句会容易理解一点：we see the old moon by the light which don't（don't?） come from the sun but from the earth.
by the light 通过光线.
不用的话,句子意思就变了但是谓语部分为毛是don't捏?应该是doesn't啊.