裤子由两条腿构成,鞋由一双组成,没有人只卖一条裤腿和一只鞋,所以必须用复数 trousers / pants 和shoes。
How much are the trousers (shoes)?
How much did you pay for the trousers (shoes)?
How much do the trousers (shoes) cost?
英语中,凡是成对构成的名词都得以复数形式出现,另如:scissors(剪刀),forceps(镊子)、pliers(老虎钳),compasses(圆规),等等。