We use comparative adjectives when talking about two things (not three or more things).

In the example below, "bigger" is the comparative form of the adjective "big":

A1   A2
A1 is bigger than A2.

Formation of Comparative Adjectives

There are two ways to make or to "form" a comparative adjective:

Short adjectives: add -erexamples1-syllable adjectives | old, fast
2-syllable adjectives ending in -y | happy, easy
RULE: add "-er" | old → older
Variation: if the adjective ends in -e, just add -r | late → later
Variation: if the adjective ends in consonant, vowel, consonant, double the last consonant | big → bigger
Variation: if the adjective ends in -y, change the y to i | happy → happier

Long adjectives: use moreexamples2-syllable adjectives not ending in -y | modern, pleasant
all adjectives of 3 or more syllables | expensive, intellectual
RULE: use "more" | modern → more modern
expensive → more expensive