为什么不能说an illness boy 而要 a sick boy?illness是形容词呀

The boy had a strange illness,which u_____ his parents illness ill sick 的 区别 ill与sick,disease,illness have an illness对不对?illness可数吗? have an illness 还是 have illness an illness还是the illness对? 为什么不能说 a 8 years’old boy? 为什么不能说 how clever a boy he is 为什么要说the/a two-year-old boy 不能说two years old boy disease,illness,sickness,ill,sick的区别 ill,sick,sickness,illness的区别 ill,illness,disease,sick有什么差别