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