Most one syllable adjectives have their comparative and superlative forms ending in -er and -est. Some two syllable adjectives also have comparative and superlative forms ending in -er and -est.

A few one syllable adjectives have irregular comparative and superlative forms.

Good (positive) / better (comparative0 / best (superlative)
Bad / worse / worst
Ill / worse / worst
Far / farther / farthest
Far / further / furthest
Far / farther / farthest
Old / older / oldest; old / elder / eldest

The determiners little and much/many have irregular comparatives and superlatives.

Little / less / least
Much / more / most
Many / more / most

Few has two possible comparatives and superlatives: fewer/less and fewest/least

Two-syllable adjectives

Adjectives ending in -y have comparative and superlative forms ending in -ier and -iest.

Happy / happier / happiest
Easy / easier / easiest

Some other two syllable adjectives ending in an unstressed vowel also have comparative and superlative forms ending in -er and -est.

Narrow / narrower / narrowest
Simple / simpler / simplest
Clever / cleverer / cleverest
Quiet / quieter / quietest

With adjectives ending in -ing, -ed, -ful and -less, only more/ most is possible.

Beautiful / more beautiful / most beautiful
Careless / more careless / most careless

With many two-syllable adjectives, both -er/-est and more/most are possible.

Longer adjectives

Adjectives of three or more syllables have comparative and superlative forms ending in more and most.

Intelligent / more intelligent / most intelligent
Practical / more practical / most practical

Words like unhappy, untidy etc., are exceptions to this rule.

Unhappy / unhappier / unhappiest
Untidy / untidier / untidiest

Compound adjectives like well-known and good-looking have two possible comparative and superlative forms.

Good looking / more good-looking (or better-looking) / most good-looking (or best-looking)