The rules for comparisons are quite simple but even native speakers seem to make mistakes.
three syllable or more adjectives : put 'more' in front
expensive ----- more expensive
two syllable adjectives not ending in 'y' : put 'more' in front
stupid ----- more stupid
two syllable adjectives ending in 'y' : replace the 'y' by 'ier'
happy -----happier
one syllable adjectives ending in one vowel and one consonant : double the consonant and add 'er'
big -----bigger
other one syllable adjectives : add 'er'
tall -----taller
Exceptions
good -----better
bad -----worse
far -----further/farther
old -----older/elder
little -----less
With a few exceptions, adverbs normally add 'more'
slowly -----more slowly
easily -----more easily
These are the exceptions
early -----earlier
late -----later
fast -----faster
hard -----harder
near -----nearer
soon -----sooner