COMPARATIVE FORMS OF ADVERBS

add -er or -est to short adverbs: 

Adverb       Comparative      Superlative

hard              harder                 the hardest
late               later                     the latest
fast               faster                   the fastest

Example: 

Jim works harder than his brother. 
Everyone in the race ran fast, but John ran the fastest of all. 

with adverbs ending in -ly, use more for the comparative and most for the superlative:

Adverb       Comparative    Superlative 

quietly         more quietly        most quietly
slowly          more slowly        most slowly
seriously      more seriously     most seriously
  
Example: 

The teacher spoke more slowly to help us to understand. 
Could you sing more quietly please? 

Some adverbs have irregular comparative forms:

Adverb        Comparative      Superlative 

badly            worse                    worst
far                farther/further       farthest/furthest 
little             less                        least
well              better                     best

Example: 

The little boy ran further than his friends. 
You're driving worse today than yesterday !

BE CAREFUL! Sometimes 'most' can mean 'very':

We were most grateful for your help 
I am most impressed by this application.