因为do 这个词（does是第三人称单数形式），大体上从两方面来讲。
1.作为行为动词，跟其他动词一样，如study, play等。他的意思是“做”等。如：
We usually do our homework in the afternoon.
He does his homework in the evening.
2.作为助动词，也就是帮助主要动词构成否定和疑问等。
We don't like bananas.
Do you like apples?
Does he like English?
He doesn't like swimming.
上面有一句He does his homework in the evening.要是变成否定句就成了这个样子：
He doesn't do his homework in the evening.
这里加上了助动词doesn't，而原来的主动词does变成原形do了。
追问我们老师说不用do(What do they have?)的do，而用does，编成好记的原因，口决(回家作业可以不用做),求您解答。我才小学五年级。(译林5b第8单元C，D部分)
追答抱歉，我初2了，不知道5b第8单元C，D部分的答案
来自：求助得到的回答