To make the comparative form of adjectives (like 'bigger' or 'more expensive') and the superlative form (like 'biggest' or 'most expensive'), first we need to know how many syllables are in the adjective.
ADJECTIVES WITH ONE SYLLABLE:
Usually if an adjective has only one syllable, we add 'er' to make the comparative form. We add 'est' to make the superlative form.
-clean → cleaner / cleanest
-cold → colder / coldest
-small → smaller / smallest
-young → younger / youngest
-tall → taller / tallest
There are some spelling changes. If there is one vowel followed by one consonant at the end of the adjective, we often double the consonant.
-wet → wetter / wettest
-big → bigger / biggest
-hot → hotter / hottest
-thin → thinner / thinnest
If the adjective ends in 'y', this often changes to 'i'.
-dry → drier / driest
If the adjective ends in 'e', we don't add another 'e', just 'r'.
-nice → nicer / nicest
-large → larger / largest
Even when the adjective has only one syllable, it's still not wrong to use 'more' or 'most'. It's possible to say 'more wet' or 'most tall'. This isn't incorrect.
There are a few adjectives that we have to use 'more' or 'most' with, even though they only have one syllable. We CAN'T add 'er' or 'est'.
fun → more fun / most fun (NOT funner / funnest)
real → more real / most real (NOT realer / realest)
right → more right / most right (NOT righter / rightest)
wrong → more wrong / most wrong (NOT wronger / wrongest)
ADJECTIVES WITH TWO SYLLABLES:
For adjectives with two syllables we generally use 'more' or 'most'.
-careful → more careful / most careful
-bored → more bored / most bored
But some two syllable adjectives can take 'er' or 'est'. It's also fine to use 'more' (for the comparative) or 'most' (for the superlative).
-clever → cleverer / cleverest
-simple → simpler / simplest
-narrow → narrower / narrowest
-quiet → quieter / quietest
Adjectives with two syllables that end in 'y' usually can add 'er' or 'est' (y generally changes to i). It's also fine to use 'more' or 'most'.
-dirty → dirtier / dirtiest
-pretty → prettier / prettiest
-happy → happier / happiest
-ugly → uglier / ugliest
ADJECTIVES WITH MORE THAN TWO SYLLABLES:
Adjectives with more than two syllables can only make their comparative by using 'more' and their superlative by using 'most'.
-beautiful → more beautiful / most beautiful
-intelligent → more intelligent / most intelligent
-interesting → more interesting / most interesting
-expensive → more expensive / most expensive
IRREGULAR ADJECTIVES:
There are also some irregular adjectives. We just need to learn these forms.
-good → better → best
-bad → worse → worst
-far → further → furthest
-little → less → least
-much → more → most