Adverbs describe the way the action of a sentence is performed. They are almost always in the -ly form. But be careful: just seeing -ly is not sufficient to call something an adverb. (For example, “lately," is a time preposition, meaning “recently.") Adverbs will usually come after a simple verb, after the object, or in the middle of a compound verb. Of course, there are exceptions.
I work quickly on the computer.
(My action of working is quick.)
He listens carefully so he doesn't miss anything important.
(His action of listening is careful.)
My friend speaks English and Chinese perfectly.
(My friends' action of speaking is perfect.)
Adverbs can also be used before adjectives.
This is a terribly expensive restaurant.
He was incredibly fast at computerized tests.
I am reasonably good at grammar.
And they can come before the past participle (as a modifier).
The woman was seriously injured in the riot.
He is rarely occupied at this hour.