请问"She rans much faster than her sister does."一句中的does 是必须的吗

an是run的过去式,过去式不用加第三人称单数.She runs much faster than her sister does才是对的.does是必须的.这句话中的than表明这是一个比较句式,比较句的前后句子成分必须相同,这句话的完整形态是She runs much faster than her sister runs,she runs 和her sister runs的句子成分才是相同的,这里的runs可以用does代替.如果分句中动词短语很长的话后面一句话用does替代就就很简洁,这句话比较简单没有体现出来.