why do we only form adjectives as adverb of manner

- An adjective describes a noun, an adverb of manner describes 'how' an action is done. Adverbs tell in what way someone does something.
- Adjectives are modified to become adverbs of manner. For example:

 

Adjective	Adverb
slow	slowly
beautiful	beautifully
careless	carelessly
smart	smartly

- Some adverbs keep the same form as adjective. For example: fast, hard, etc.

- Some adverbs are formed by adding suffixes. For example: He moved the dial clockwise.