Why is it that some words you say "X-er" and for others you say "more X"?

Question

more fun instead of funner
bigger instead of more big

Answer 1

browneyedbeauty has the basic pattern correct -- LONGER adjectives use "more", adjectives of one-syllable, or two-syllables ending with y and a few others simply add -er (and -est for the superlative).

The one example you give -- of fun-- is a major exception. So why is THIS word treated as it is?

The reason is that the word (unlike "simple" and "fair") was, until recently, NOT an adjective.

When you use a noun for a comparative or superlative expression you do NOT change the form of the noun but simply use the words "more" and "most". (In other words, -er and -est are ADJECTIVAL endings.)

You can see this difference -- and that "fun" is GENERALLY handled as a noun-- by looking at the following constructions.

First, just you say "it was a lot more trouble" and "that was the most trouble" you can say "I had more fun this time" or "That was the most fun I've every had." In all of these, the key word is functioning as a noun.

Compare the expressions "That was so much fun/trouble !" and "That was a lot of fun/trouble". "So much" and "a lot of" are used with nouns. If the words were being treated as an adjective, you would say "That was so fun/trouble!" and "That was very fun/trouble!" (I suppose some might now use some of these, but "so much" and "al lot of" are the older and more generally acceptable forms to use.)


So, even when "fun" shifts over to be used as an adjective, or when the lines are blurry, several of the constructions you use it with, including the comparative and superlative take the form appropriate to NOUN constructions.

http://www.worldwidewords.org/qa/qa-fun1.htm
http://www.ling.ed.ac.uk/linguist/issues/9/9-795.html

note that the main definitions are as a NOUN, and the use as an adjective is "informal" (because it's a recent innovation)
http://www.thefreedictionary.com/fun

Answer 2

For one syllable words like "big" or "green" you add the -er as well as for two syllable words that end in "y" like "pretty" and "funny."

All other two or more syllable words needto have "more" added because the words themselves are "too long" to have an extra syllable added. (beautiful, delicious, colorful, truthful, useful, etc.)