Consider this sentence: Sue is such a fun person to be with, but I think Jim is more fun than Sue. However, I think Ted is the most fun. We don't say "funner than" or "the funnest" although these are apparently slowly becoming acceptable. This is because English is a living language and continuous to evolve (slowly change).
I've never really considered or even wondered why fun doesn't have the same comparative or superlative forms as other adjectives. I've just thought of it as one of the many exceptions to the rules in English. I simply point it out that unlike most one-syllable adjectives that get an 'er' or an 'est' suffix, the word fun is just different--for some reason. So while tall becomes taller and the tallest, and thin becomes thinner and the thinnest, fun becomes more fun and the most fun. "It's just English," I would say, to which my ESL students would usually chuckle (laugh lightly) in understanding.
What I didn't know until recently is that fun was once only used as a noun! Wow! Who could have known? Well, apparently, a lot of people especially those born before 1970, as Grammar Girl points out. Fun has only relatively recently been used as an adjective and is still not completely accepted as such by strict grammarians. This makes complete sense! This explains why we say "more fun" and "the most fun" just like we say "I want more soup," or "He ate the most pie for desert."
While most of us have no problems saying, "That was a fun party," or "We had a fun class," it still sounds strange to say "This game is funner than that one," or "Tom was the funnest person at the party." It sounds much better to say "This game is more fun than that one," and "Tom was the most fun at the party."
As I mentioned earlier, funner and the funnest are slowly becoming acceptable. For now, however, say "more fun" and "the most fun." They sound much better to most people including your strict grammar teacher.
Thanks for checking out this small lesson. Be sure to like the small guide site on Facebook, and follow me on Twitter @joeyu2nd. Have fun using English, and catch you next time.