Learn Finnish: Juhlasalin perimmäinen ovi on kiinni. - The rearmost door of the banquet hall is closed.

It is built from the noun perä (the rear, back, or end of a space). By adding the extremity suffix -immäinen, it becomes an adjective meaning 'the one furthest at the back' or 'rearmost'. Notice that the final -ä of perä drops out when attaching the suffix.

Could I use taaimmainen instead of perimmäinen here?

You could, as both can translate to 'rearmost'. However, perimmäinen is much more natural here because it refers specifically to the deepest part or the very back of a contained space (like a hall or a room). Taaimmainen (from the root for 'behind') is better suited for the last item in a line or sequence.

Why is the door described as kiinni rather than suljettu?

Kiinni is a fixed state word meaning 'closed' or 'shut', and it is the most standard, natural way to describe a door or window in everyday Finnish. Suljettu is the passive past participle of the verb sulkea (to close). While suljettu ovi (a closed door) is grammatically correct, using it with the verb olla (ovi on suljettu) places slightly more emphasis on the action that someone closed it, rather than just its static current state.

Why is juhlasalin written as a single compound word rather than two separate words?