Fast and hard are both adjectives and adverbs:
fast -> fast hard -> hard
Maria is a fast learner. (fast = adjective)
Maria learns fast. (fast = adverb)
James is a hard worker. (hard = adjective)
James works hard. (hard = adverb)
We form the comparative of regular adverbs with more or less:
carefully -> more/less carefully
You should do your work more carefully.
She dose her work less carefully than other people.
The comparative of well is better:
She speaks Arabic better than me.
The comparative of fast and hard are faster and harder:
Could you walk faster? We are in hurry.
You will have to work harder in future.