table manners的谓语动词用复数，因为manners是复数，例如：

He felt awkward the first time he had dinner in his American host family，because he didn't know hiw to use knief and fork. So， I say that table manners are very important.他第一次在美国的寄宿家里吃饭时，和尴尬，因为不知道怎样使用刀叉。所以，我说餐桌礼仪很重要（本句中，table nanners的谓语动词用了复数are）