Here's how it works:
* Adjective + -ily = Adverb
Examples:
* Happy (adjective) + -ily = Happily (adverb)
* Slow (adjective) + -ily = Slowly (adverb)
* Gentle (adjective) + -ily = Gently (adverb)
* Quiet (adjective) + -ily = Quietly (adverb)
It basically answers the question "how?"
* She walked happily down the street.
* He spoke slowly and carefully.
* The wind blew gently through the trees.
* The cat moved quietly across the room.
There are a few exceptions, and some adjectives form adverbs with "-ly" instead of "-ily" (e.g., friendly, lovely, lonely).