1. how much the coat costs在句中做ask的宾语从句,即ask sb sth中的sth；
而宾语从句的句序为“引导词how much + 陈述句”,所以写成the coat costs这样的主谓陈述句序.
再如：I want to know how old you are.
2. 如果加does,一则后面的costs要还原成动词原形cost,二则它只能做一个单独的疑问句,即：How much does the coat cost?
它不能在复合句中充当从句.
3. does the coat cost是所谓的疑问句序,或者语法书上所谓的倒装句序.