Comparatives

Addinf -er
All words with one syllable(smal, cheap, old, fast)
All words with two syllables ending in -y; -e; -er; -le; -ow
(pretty, severe, clever, simple, narrow)

Adding more than

All words with more than one syllable(except the ones above)

Irregular

good - better
bad - worse
far - further/farther
much - more
many - more
little - less