fun的比较级为什么加more

因为fun为名词，故比较级用more，more fun中文意思是

更有趣;美国人用;更多乐趣;更加有趣;有趣得多;

例句

What could be more fun than a gift that keeps coming through the letterbox every month?

有什么比每个月都从信箱里取出礼物更有趣的呢?