Learn Finnish: Me jätimme kirotun arkun luolaan. - We left the cursed chest in the cave.

This is the main quirk of verbs like jättää (to leave something). In English, you leave something in a place. In Finnish, the action of leaving is seen as a movement ending into that place. Therefore, it requires the illative case (the "into" case), making it luolaan.

Why do kirotun and arkun end in -n?

They form the direct object of the sentence (the cursed chest). Because the action of leaving the chest is a completed, whole action, the object takes the accusative case. For singular nouns, this looks identical to the genitive case ending in -n. Both the noun (arkku) and its describing adjective (kirottu) must match in case.

Why does jätimme only have one t when the dictionary form is jättää?

This is consonant gradation. Adding the personal ending -mme closes the syllable. A closed syllable forces the strong double tt of the basic verb to weaken into a single t.

What kind of word is kirotun?

It is the passive past participle of the verb kirota (to curse), functioning here as an adjective meaning "cursed". Just like a normal adjective, it takes the same ending as the noun it describes, which is why kirottu becomes kirotun to match arkun.