Adjectives vs. Adverbs
He is a slow runner.   > In case you want to further describe a noun, an adjective is needed.
He runs slowly.          > In order to further describe a verb, an adverb is needed.
He is an extremely slow runner.    > In order to further describe an adjective, an adverb is needed.
He runs extremely slowly.    > In order to further describe an adverb, another adverb is needed.
In most cases the suffix LY is added to the adjective. e.g. slow - slowly
However, there are some exceptions:
1)   a silent e is dropped, e.g. true - truly
2)   y turns into i, e.g. happy - happily, angry - angrily.
3)   le after a consonant is dropped, e.g. sensible - sensibly, probable - probably
4)   after ll only add y, e.g. full - fully
5)   adjectives ending wit ic add ally, e.g. fantastic - fantastically, basic - basically
6)   adjectives ending with ly use in a ... manner / way, e.g. friendly - in a friendly manner
Irregular Adverbs:
good > well
hard > hard    (hardly = seldom)
late > late    (lately = recently)
short > short   (shortly = soon)
near > near   (nearly = almost)
Some adjectives are used as adverbs without modification, e.g. daily, weekly, monthly, yearly, early, much, fast.