Adverbs are usually formed by adding -sti to the inflecting form of the corresponding adjective.

Adjective Adverb English
helppo helposti easily
hidas hitaasti slowly
huono huonosti badly
kaunis kauniisti beautifully
kiire kiireesti hastily
mahdollinen mahdollisesti possibly
nopea nopeasti quickly
selvä selvästi clearly

The great thing about adverbs like these is that they don't inflect!

Apart from adverbs ending in -sti, you have three large groups:
- Adverbs of time: huomenna, eilen, tänään, joskus, usein, kerran, heti, pian,...
- Adverbs of place: kaukana, alta, luokse, eteen (all the postpositions are in fact adverbs)
- Adverbs of quantity: paljon, hiukan, vähän, vain, melkein,...

When dealing with adverbs of place, you DO inflect the adverb, but only in the location cases. Visit the page about postpositions to find out more about this.