An adjective only modifies nouns (or pronouns):
She is a beautiful girl.
The girl is beautiful.
She is beautiful.
An adverb modifies anything but nouns (or pronouns):
She sings beautifully. (verb + adv.)
She sings incredibly beautifully. (adv. + adv.)
His job is undeniably important. (adv. + adj.)
Jim is standing right behind the tree. (adv. + prep. phrase)
Undeniably, not everyone needs the product. (adv. + sentence)
❌ The girl is beautifully.
❌ She is a beautifully girl.
An adverb is equivalent to a prepositional phrase.
She lives here. = She lives in New York.
I am home. = I am at home.
Smith waited for me downstairs. = Smith waited for me on the lower floor.
A prepositional phrase can be used to modify a noun, but it has to be a post-modifier in this case:
People in New York were not as friendly as I had thought.
The couple on the lower floor had a fight.
Likewise, an adverb can also be used to modify a noun, but it has to be a post-modifier in this case:
People here were not as friendly as I had thought.
The couple downstairs had a fight.
Sometimes, an adjective and an adverb can be the same word:
The mountain is high (adj).
We jumped high (adv.).
Some adverbs have two forms, but they do not convey the same meaning:
We jumped high (adv., indicating the vertical distance).
❌ We jumped highly.
People think highly (adv., indicating the degree or level) of the mayor.
❌ People think high of the mayor.
Not every word ending with -ly is an adverb:
In most cases, adjective + ly → adverb:
exact (adj.) → exactly (adv.)
happy (adj.) → happily (adv.)
economic (adj.) → economically (adv.)
economical (adj.) → economically (adv.)
However, noun + ly → adjective / adverb:
friend (noun) → friendly (adj.)
time (noun) → timely (adj.)
hour (noun) → hourly (adj. / adv.)