为什么what great fun it is 这个句子里,great前没有a?

为什么what great fun it is 这个句子里,great前没有a? 为什么是what great fun it is!而不是what a great fun it is! What great fun it was! fun前可不可以加a?为什么? What great fun it was!fun前可不可以加a?为什么? 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 请问那个正确 What a great fun it is to go surfing; What great fun 英语的fun可数吗?是What a great fun it is!还是What great fun it is! What ------great fun to read ------interesting book it is!A what a great fun/what great fun为什么不可以用how great fun Your friend is a great fun.这个句子对吗 为什么不能说it is a great fun?