We have seen that adjectives of one syllable usually form their comparative and superlative forms by the addition of –er and –est to the positive. There are, nevertheless, a few exceptions to this rule.

When the positive adjective ends in a consonant + y, the y is changed into i before adding –er and –est.

Happy (positive), happier (comparative), happiest (superlative)
Easy, easier, easiest
Heavy, heavier, heaviest

When the positive adjective is of one syllable and ends in a short vowel + single consonant, this consonant is doubled before adding –er and –est.

Red (positive), redder (comparative), reddest (superlative)
Big, bigger, biggest
Hot, hotter, hottest
Fat, fatter, fattest

Adjectives of more than two syllables form the comparative and superlative by putting more and most before the positive.

Examples are:

Careful (positive), more careful (comparative), most careful (superlative)
Difficult, more difficult, most difficult
Beautiful, more beautiful, most beautiful

Two syllable adjectives ending in –ful, -less, -ing and –ed take more and most before the positive.

Hopeless, more hopeless, most hopeless
Boring, more boring, most boring
Surprised, more surprised, most surprised
Useful, more useful, most useful

Notes

The comparative in –er is not used to compare two qualities of the same person.

John is more smart than prudent. (NOT John is smarter than prudent.)
Here we are comparing two qualities – smartness and prudence – of the same person.

Peter is more brave than intelligent. (= The bravery of Peter is greater than that of his intelligence.)
But the comparative in –er is used to compare the same quality present in two people or things.

John is smarter than Peter. (Here we are comparing the smartness of John with that of Peter.)
Alice is older than Mary. (Here we are comparing Alice’s age with Mary’s age.)