Are you good at dangcing?这里的为什么要用Are?

Are you good at dangcing?这里的为什么要用Are? that's what you are good at这里为什么要加what?类似的还有you are what you you and i are good friends.这里为什么用are? 这里为什么要加ing?Are they good at helping you? Thay are at a good price.这里为什么用的是at? You are very good at telling strories 翻译意思,为什么要用at tell为什么要加 Are you good at ________? Are you good at swimming? you,at,are,good,swimming what are you good at Are you good at Chinese Are you good at ____