为什么不能说it is a great fun?

为什么不能说it is a great fun? 为什么是what great fun it is!而不是what a great fun it is! A.It is of great fun. 为什么what great fun it is 这个句子里,great前没有a? 为什么能说What fun it is to do sth.却不能说How fun it is to do sth. What a great fun it is to go surfing; What great fun it is t what a great fun it is to go fishing和what great fun it is to great fun ,但不是说 抽象名词有修饰词,可以加不定冠词吗?如 a great success为什么不能写成 英语的fun可数吗?是What a great fun it is!还是What great fun it is! 请问那个正确 What a great fun it is to go surfing; What great fun I think it is a great fun.改错) _____ great fun it is to fly in a balloon!