1. It's important to keep a promise.
2. It's fun to play tennis.
Looking at the two sentences above, we can see both "important" and "fun" are adjectives. What are the comparative and superlative forms of "fun"?
Since it's a short adjective with a single short vowel, it should be, according to the general rule, funner and funnest, as in "redder, reddest", but I don't think they are standard usage yet. Then "more fun" and "the most fun" would be acceptable?