Comparative and Superlative forms of adjectives -

Some two-syllable adjectives which have both kinds of comparative and superlative forms are:
 

clever → cleverer/more clever → cleverest/(the) most clever

common → commoner/more common → commonest/(the) most common

gentle → gentler/more gentle → gentlest/(the) most gentle

humble → humbler/more humble (etc.)

hollow → hollower/more hollow

narrow → narrower/more narrow

polite → politer/more polite

quiet → quieter/more quiet

simple → simpler/more simple

stupid → stupider/more stupid

subtle → subtler/more subtle

For two-syllable adjectives ending in "e", like "little", only add "r" for the comparative form and "st" for the supperlative:

 

Irregular forms are: 

 good                        -     better   -   best

bad                           -    worse    -   worst

 far                           -     farther/further    - farthest/furthest **

many, some, much    -     more   -     most 

little                          -     littler / less  - littlest / least

** When shall we use farther and when further?

I found the following explanation which I think is very helpful:

"Further" Versus "Farther" The quick and dirty tip is to use “farther” for physical distance and “further” for metaphorical, or figurative, distance. It's easy to remember because “farther” has the word “far” in it, and “far” obviously relates to physical distance.