In addition to simple adverbs such as heti “immediately” and melko (rather), there is a large, potentially infinite number of adverbs derived from adjectives.
The most common suffix in adverbs is sti, which typically means “in a … manner”, where “…” denotes the meaning of the base word. The sti suffix is much like the “ly” suffix in English and can be appended to the inflection stem of almost any adjective. E.g. from rohkea (brave) we get rohkeasti (bravely) and from kaunis : kaunii|n (beautiful), we get kauniisti. The suffix sti has boundary gemination, so e.g. rohkeasti tänne is pronounced as rohkeastit tänne in standard language.
Another common suffix in adverbs is in, e.g. hyvin (well; very), from hyvä (good). By its form, it is originally the instructive plural of the adjective. Such derivations are often based on comparative forms, e.g. from kauniimpi (more beautiful, from kaunis, beautiful) we get kauniimmin (more beautifully), and from the corresponding superlative kaunein : kauneimman we get kauneimmin (most beautifully).