We use an adverb to say how an action is performed.

He speaks English fluently.
She answered correctly.

We use an adverb to add information about the time/place/manner.

How long have you lived here?

We can use an adverb to add information to an adjective.

She was extremely happy to see him again.
I really hate travelling by train.

The majority of adverbs are formed by adding ly to an adjective.
There are some exceptions - irregular adverbs.

nicely, quickly, beautifully, happily, economically

If the adjective ends in ic we add ally.

basic - basically, dramatic - dramatically

Some adverbs have the same form as adjectives.

fast, daily, late, early, hard

The camera works perfectly.

We use an adverb
(perfect - adjective,  perfectly - adverb) when we want to say how we do something.

Did you work hard today?

See the list of irregular adverbs.

Her behaviour was silly.
She behaved rather stupidly.

Some words ending in -ly are adjectives (friendly, likely, lonely, ugly) and cannot be used as adverbs.

His answer sounded correct.
He looks happy.

After 'linking' verbs we use adjectives not adverbs.