Noun: extinguisher, fire extinguisher.
Sammutin comes from sammuttaa, a transitive verb (takes an object) that means ”to extinguish,” ”to put out” ”to shut down” or ”to turn off” + the deverbal suffix –in, which here makes it into instrumental noun.
Sammutin is formed in the same way that the noun avain (”key”) is formed from avata (”to open”) and the noun puhelin (”telephone”) is formed from the verb puhella (”to chatter”).
Let’s translate the whole sign, ”Sammutin oven takana,” or ”Fire extinguisher behind the door.”
Oven is the genitive singular of the noun ovi (”door.”)
Takana is an adverb that means ”behind.” When takana is used, the person or thing it is describing is always in the genitive.