“How many days are there in a week?”这句话为什么要加“are there”?

还原成陈述句,
there are 7 days in a week
然后,你对7 进行画线部分提问,就得到此句
规则是：首先确定特殊疑问词,是how many days,然后,原句的there are ,的are 提前,就得到
How many days are there in a week
如果还有疑问,比如,为何要加be 动词,欢迎追问