Why and when do we use the word 'more' instead of adding an 'er' to the end of a word... for example, we say more fun rather than funner...

These are two separate adjectives, with different meanings.,

funny is an adjective, meaning basically 'it makes you laugh'.
funnier Fine
funniest Fine

fun is a noun that many people, especially the young, have started to use as an adjective. Basically, it means 'you enjoy it'. As an adjective, it is on the way to being accepted, but is still substandard.
funner Not on the way to being accepted
funnest Not on the way to being accepted

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We use 'more' when we are comparing an adjective that has two syllables or more.
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That answer makes me more happy than other answers on this topic. D'oh! Clearly there are exceptions to this rule, or the rule is wrong.
"Fun" is not an adjective, but a noun.
"more" can also be used with nouns:
My brother has more cookies than my sister.
"More fun":

They had more fun at the park than we did because when we went, it rained. We were cold and wet. That was not fun at all.
Fun is both an adjective and a noun. "That was a fun party." or "That's not my idea of fun."

So you can have more fun at the funner parter and most fun at the funnest party. So depending on the context, both are correct.
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Some people still resist the idea of 'fun' as an adjective. I wouldn't recommend its use in a setting where you want to use good and careful English.

Best wishes, Clive
however... I am excited about this. In fact, I am exciteder than you.
Ok...i´m sorry... but "funner" party?

Is this a joke?

It is supposed to be "funnier".

And by the way.... "funnest" party?

Where did you see that?

Its FUNNIEST for God´s Sake!!
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 Clive's reply was promoted to an answer.
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