Learn Finnish: Me vierailimme eilen isoissa palatseissa ja kartanoissa. - We visited big palaces and manors yesterday.

In English, you just visit a place as a direct object, but in Finnish, the verb vierailla (to visit) requires you to visit in a place. Since we are visiting multiple palaces and manors, we use the inessive plural ending -issa.

Why is the adjective isoissa also in the inessive plural?

Finnish adjectives must agree with the nouns they modify in both number (singular/plural) and case. Because palatseissa (palaces) and kartanoissa (manors) are in the inessive plural, the adjective iso (big) has to match them, becoming isoissa.

Where does the e come from in palatseissa if the dictionary word is palatsi?

For many words ending in -i, the final -i changes to an -e- before you add the plural marker -i-. So, palatsi becomes the plural stem palatsei-, to which you add the inessive ending -ssa to get palatseissa.

Why don't the vowels change in isoissa and kartanoissa before the plural -i-?