What is an adverb?

Adverbs are often thought of as words that modify verbs, and this is indeed the role they usually have in a sentence. Here are some examples:

“The cat crept quietly down the street.” (Quietly is an adverb modifying the verb crept)

"My stomach hurt so I ran slowly." (Slowly is an adverb modifying the verb ran)

"To boldly go where no one has gone before." (Boldly is an adverb modifying the verb go)

However, adverbs can also modify adjectives, other adverbs, phrases, and even entire sentences. Here are some more examples:

“I’m not quite finished.” (Quite is an adverb modifying the adjective finished)

“Cheetahs can run incredibly fast.” (Incredibly is an adverb modifying the adverb fast)

“She stopped by the store briefly to pick up a few things.” (Briefly is an adverb modifying the phrase stopped by the store)

“Unfortunately, it’s supposed to rain on Friday.” (Unfortunately is an adverb modifying the entire sentence)

It may be helpful to think of adverbs as words that answer certain questions a reader or listener might have.

The train is arriving soon. (Soon is an adverb answering ‘when does the train arrive?’)

I like to shop locally. (Locally is an adverb answering ‘where do you like to shop?’)

We are getting along fabulously. (Fabulously is an adverb answering the question ‘how are you two getting along?’)

It’s constantly raining in this city. (Constantly is an adverb answering the question ‘how often does it rain?’)

When they begin a question, as they do above, when, where, and how (in addition to why) are also adverbs!

Most adverbs are formed by adding -ly to an adjective (quaintly, awkwardly, honestly), but there are many common adverbs that do not end in -ly, including again, also, just, never, often, soon, today, too, very, and well.

Other parts of speech:
What is an adjective?
What is a noun?
What is a preposition?
What is a verb?

Other articles you may be interested in:
The adverb: a most fascinating POS
What is a flat adverb?
Should you never use adverbs?