Comparison with -er/-est

clean - cleaner - (the) cleanest
We use -er/-est with the following adjectives:

1) adjectives with one syllable

cleancleanercleanest
newnewernewest
cheapcheapercheapest

2) adjectives with two syllables and the following endings:

2 - 1) adjectives with two syllables, ending in -y

dirtydirtierdirtiest
easyeasiereasiest
happyhappierhappiest
prettyprettierprettiest

2 - 2) adjectives with two syllables, ending in -er

clevercleverercleverest

2 - 3) adjectives with two syllables, ending in -le

simplesimplersimplest

2 - 4) adjectives with two syllables, ending in -ow

narrownarrowernarrowest

Spelling of the adjectives using the endings -er/-est

largelargerlargestleave out the silent -e
bigbiggerbiggestDouble the consonant after short vowel
sadsaddersaddest
dirtydirtierdirtiestChange -y to -i (consonant before -y)
shyshyershyestHere -y is not changed to -i.
(although consonant before -y)

B - Comparison with more - most

difficult - more difficult - (the) most difficult
all adjectives with more than one syllable (except some adjectives with two syllables - see
2 - 1 to 2 - 4)

C - Irregular adjectives

goodbetterbest 
badworseworst 
muchmoremostuncountable nouns
manymoremostcountable nouns
littlelessleast 
littlesmallersmallest 

D - Special adjectives

Some ajdectives have two possible forms of comparison.
commoncommoner / more commoncommonest / most common
likelylikelier / more likelylikeliest / most likely
pleasantpleasanter / more pleasantpleasantest / most pleasant
politepoliter / more politepolitest / most polite
simplesimpler / more simplesimplest / most simple
stupidstupider / more stupidstupidest / most stupid
subtlesubtler / more subtlesubtlest
suresurer / more suresurest / most sure

Difference in meaning with adjectives:

farfartherfarthestdistance
furtherfurthestdistance or
time
latelaterlatest 
latterx 
xlast 
oldolderoldestpeople and things
eldereldestpeople (family)
nearnearernearestdistance
xnextorder