Adjective: clean, neat, wicked, cool.
Siisti appears in our example in the translative case, siistiksi.
Siisti is derived from a Russian word, чистый (čístyj), which means ”clean.”
The related Finnish verb is siistiä, which means ”to clean up.”
Let’s translate the whole sign, ”Jätäthän keittiön siistiksi seuraaville, kiitos,” or ”Please leave the kitchen clean for the next ones, thanks.”
Jätäthän is constructed from the 2nd person singular of the verb jättää (a transitive verb meaning ”to leave”) with the particle suffix –hän appended. The particle –han can be appended to a verb to indicate a wish or to politely ask someone to do something instead of using the imperative. (The back vowel equivalent is –han.) Jätäthän might be translated ”Please leave…” or ”Leave…if you would?”
Keittiön is accusative singular of keittiö which means kitchen.
Siistiksi is in the translative case. The translative case indicates a change in state (like with the English meaning ”change into”) as opposed to a change in location. Note that jättää is a verb that always is paired with one of the ”lative” cases such as illative, ellative, allative, etc.
Seuraaville is the allative plural of seuraava, an adjective meaning ”next” or ”following.” The allative has the basic meaning of ”onto,” but here it best translates as ”for the next” and it’s implied that it means ”for the next ones.”
Kiitos means ”thanks” and is probably one of the very first words you learned in Finnish. (It was for me! From a bartender outside the Central railway station in Helsinki in the summer of 2009.)