Adjectives with one syllable always add -er to show the comparative and -est to show the superlative.
big, bigger, biggest
Two-syllable adjectives ending in -y also use -er, and -est after changing the -y into an -i, like this:
easy, easier, easiest
busy, busier, busiest
ready, readier, readiest
happy, happier, happiest
Two-syllable adjectives not ending in -y use more and most to show comparative and superlative:
My bedroom is peaceful.
The forest is more peaceful.
Being at the ocean is the most peaceful of all.
helpful, more helpful, most helpful
awkward, more awkward, most awkward
careless, more careless, most careless
Adjectives of three or more syllables always use more and most to show comparative and superlative.
This horse is enormous.
The bison is more enormous.
That elephant is the most enormous of all.
important, more important, most important
wonderful, more wonderful, most wonderful
different, more different, most different
Two-syllable adjectives ending in -y change the -y to -i before adding -er and -est for comparative and superlative. Other two and three-syllable adjectives use the words more and most to show comparative and superlative.