- Derives adverbs.
From Proto-Finnic *-stik, probably equivalent to -sta (elative suffix) + -i (lative suffix). Cognate to Estonian -sti. For the use of an elative ending to derive such adverbs, compare Erzya -сто (-sto), -стэ (-ste), (-сте (-sťe)) in вадрясто (vadŕasto), парсте (parsťe, “well, good”).
- (adverbial suffix, comparable) Forms adverbs from adjectives describing the manner in which something is done; corresponds to the English suffix -ly.
- (multiplicative, not comparable) Forms multiplicative adverbs describing how many times something is done; corresponds to the number + times structure in English.
- (intensifier) Forms intensifying adverbs from certain nouns.
- (adverbial suffix):
- The -sti adverbs are used to describe the manner that something is done; the other common use in English to instead modify adjectives, such as to describe their degree or extent (tolerably quiet, exceptionally cold), is expressed in Finnish by making an adverb out of the adjective using the genitive case (siedettävän hiljainen, poikkeuksellisen kylmä).
- The suffix is appended to the weak stem, i.e. one takes the -n off the genitive singular form and adds this suffix to it. The resulting adverb is comparable, if the base word is comparable: for example kauniisti, kauniimmin (“more beautifully”), kauneimmin (“most beautifully”). It can be appended to all adjectives, but also to selected nouns and numerals.
- (multiplicative): Used only with small numbers, and with some pronouns; note that yksi (“one”) → yhdesti (“once”) is not standard Finnish (compare kerran).
- (intensifier): The suffix is often applied to a vulgar term: perkele → perkeleesti.
- (expression of how many times something happens): kertaa
- adverbial suffix