The general rule is that if the word is one syllable you add -er, if it's two syllables you can add -er or use more, if three syllables or more you can only use more. This is for the comparative, but the same rules apply for the superlative -est or most.
There are some exceptions. For example in the U.S. it was taught for a long time that adjectives ending with "n" or "ful" would not follow the above rules. So you could not for example, say "carefuler" even though it has two syllabus and you generally would have the option of -er or more. You would have to say "be more careful." as an example. The same applies to "fun" which now some people will say "funner" using the one syllable rule above, however, purists would say this needs to be said "more fun." Another exception is two syllable words ending in "y" they add "ier" such as "happier."
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