Use words ending in -er or modified by the word more to compare two items. This is known as the comparative degree.
Use words ending in -est or modified by the word most to compare three or more items. This is known as the superlative degree.
Correct: K2 is taller than Annapurna.
Incorrect: Annapurna is the taller of the three peaks.
(Three or more requires superlative.)
Correct: Annapurna is the tallest of the three peaks.
Normally, -er and -est are added to one-syllable words.
-er and -est are added to two-syllable words unless the new word sounds awkward.
Correct: fairer prettier handsomestUse the modifiers more or most with all root words longer than two syllables as well as with two syllable words that sound awkward. Always use more or most with adverbs that end in -ly.
Awkward: famousest readier
Correct: most famous more ready
Incorrect: beautifuller smoothliest
Correct: more beautiful most smoothly
Correct: friendliest beastliest (adjectives, not adverbs)
Less and least form comparisons of a lesser degree in a similar manner. Less is used when comparing two items, least with three or more.
See also Irregular Comparisons and Comparison Problems.