Comparing things

Adjectives that compare 2 things = Comparative

1 syllable -  add ER  (short -- shorter, tall--taller)
1 syllable that was spelled with 1 consonant -- 1 vowel -- 1 consonant-- repeat the last consonant to keep the short (not the name) vowel sound -- add ER  (big-bigger, thin--thinner)
2 syllables that end in ow or le  add ER (yellow-yellower, little--littler)
2 syllables that end in Y -- change the Y to I and add ER  (pretty--prettier, happy-happier)
Other 2 syllables act like
3 or more syllables:  Use the word more before the adjective.  (more modern, more beautiful, more interesting)

Superlative= Adjectives that compare 3 or more things - use the same categories, 

                      but add THE and use different suffixes (est) or helping words.(The most)

1 syllable - add EST (short - the shortest, tall - the tallest)

1 syllable CVC - 1 consonant 1 vowel 1 consonant -- double the last letter and add EST.  The Biggest, the Thinnest.

2 syllables that end with ow or le act like 1 syllable:  Narrowest, littlest

2 syllables ending in Y -- NO Y -- add iest pretty-prettiest, happy-happiest, silly- silliest

Other 2 syllables act like

3 or more syllables -- NO ENDING -- USE THE MOST before the adjective:  The most modern, the most beautiful, the most interesting.

Exceptions:  good -- better -- the best

                      bad - worse than -- the worst

                      far -- farther -- the farthest

                      fun -- more fun -- the most fun

Rules and Examples in a slideshow

Comparative

Comparative and Superlative