adjectives only modify nouns
Ex: the black cat. black is the adj modifying cat.
adverbs modify verbs, adjectives, and other adverbs.
an adverb modifying a verb ex:
"The cat ran quickly towards the door when the doorbell rang." Ran is modified by quickly, because quickly describes how the cat is running.
an adverb modifying an adjective ex:
"The cat was very black and very fat." "very" is modifying the adjectives black and fat-it describes how black and how fat the cat is.
an adverb modifying an adverb ex:
"The cat ran quite quickly away from the boy when he scared it." quite modifies the adverb "quickly" which is modifying ran.
here are some links which may help as well:
OWL at Purdue University: Adjective or Adverb?
adverbs adjectives english grammar
ENGLISH PAGE - Adverbs / Adjectives / Linking Verbs
Adverbs Page 1
a note: many adverbs end in "-ly", many adjectives end in -ive," "-ous," "-y," "-ful," or "-ent".
I hope this helps!!