Grammar - degrees of comparison

1. Adjectives of one syllable

Adjectives of one syllable add -er and -est. Adjectives ending on 'e' just add -r and -st. Some adjectives double the final consonant.   Adjective Comparative Superlative small warm old nice big smaller warmer older nicer bigger smallest warmest oldest nicest biggest

2. Adjectives of two syllables

Some adjectives of two syllables add -er and -est. Adjectives ending in 'y' use -ier and -iest. Adjectives ending in -ful, -less, -ing, -ed use more and most. Some other adjectives of two syllables use more and most (modern, famous, normal, correct, ...).   Adjective Comparative Superlative narrow happy useful boring modern narrower happier more useful more boring more modern narrowest happiest most useful most boring most modern

3. Adjectives of three or more syllables

Adjectives of three or more syllables use more and most.   Adjective Comparative Superlative beautiful dangerous exciting more beautiful more dangerous more exciting most beautiful most dangerous most exciting

4. Irregular adjectives

You will have to learn these by heart.   Adjective Comparative Superlative good (well) bad little much, many far late old better worse less more farther, further later older, elder best worst least most farthest last, latest older, oldest

 Sumber : http://www.learnenglish.be/gr2_degrees_study.htm