Does she have a sister 为什么要用have不用has?

he she it的确要用动词三单形式,但是does是助动词,助动词后面要用动词原形,即has改为have.
所以以后就要记住了那什么情况用has?举个例子吧He has a dogShe has some books等等陈述句。但是如果是否定句，有不同了，如He doesn't have a dog。She doesn't have any books。反正你记住助动词do/does/did后用动词原形