The comparative ranks one thing against another (taller, more useful), while the superlative ranks one thing against all others in a set (the tallest, the most useful). Both apply to gradable adjectives and adverbs, and English builds them in two ways: inflection (-er / -est) or the analytic forms more / most.
Syllable count is the working rule of thumb:
| Adjective type | Comparative | Superlative |
|---|---|---|
| One syllable | fast → faster | fastest |
| One syllable, single vowel + consonant | big → bigger (double the consonant) | biggest |
| Two syllables ending -y | happy → happier | happiest |
| Other two-syllable | usually more careful | most careful |
| Three+ syllables | more interesting | most interesting |
Some two-syllable adjectives (narrow, simple, clever, quiet) accept both forms. Most adverbs ending in -ly take more/most (more quickly).
A few high-frequency words are suppletive: good → better → best, bad → worse → worst, far → farther/further → farthest/furthest, little → less → least, much/many → more → most. Further tends to be preferred for abstract extension (further details), farther for physical distance, though the two overlap.
Degree modifiers grade the gap: much, far, a lot, even, rather intensify (much bigger); a bit, a little, slightly soften (a bit cheaper). Note that very modifies a plain adjective but not a comparative: very big but much bigger, not very bigger.
Only gradable adjectives compare. Ungradable adjectives describe absolute, all-or-nothing qualities (dead, unique, perfect, impossible, married) and resist comparison and degree modifiers in careful usage, since one thing cannot logically be more dead than another. Prescriptive objections to more unique rest on this distinction, though such forms occur informally for rhetorical effect.