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

为什么不能说an illness boy 而要 a sick boy?illness是形容词呀 I'm an English为什么不能这样说?书上说I'm English或I'm an English boy都可以,但是不可说I'm an English,这是为什么啊,不是有I'm Chinese么. 为什么不能说He is an English 为什么不能说he is an english? 为什么不能说 a 8 years’old boy? 为什么要说the/a two-year-old boy 不能说two years old boy An Easy Enlish question .I don't remember why she left the boy by himself .而为什么不能说I don't remember why she had left the boy by himself .PLEASE PLEASE PLEASE! have an illness对不对?illness可数吗? an illness还是the illness对? an American boy 为什么冠词用an? 为什么不能说 how clever a boy he is 可以等于What a clever boy he is! 为什么不能说he will leave after an hour?要用in an hour? His illness accounted for his absence.accounted for 为什么不能换成explained an 8-month-old boy为什么用an? Tom is the first boy to take a shower every morning.的句子成分为什么不能说Tom is the first boy takes a shower every morning. 为什么不能说哦 The boy had a strange illness,which u_____ his parentsRT, have an illness 还是 have illness 还有 have influence on 还是 have an influence on?