Learn Croatian: Trebam petnaest soba i šesnaest ključeva. - I need fifteen rooms and sixteen keys.

In Croatian, any number from 5 and up requires the noun that follows it to be in the genitive plural. That is why we use soba and ključeva after the numbers petnaest and šesnaest.

The word soba looks exactly like the singular word for "room". How do we know it is plural here?

You are right, it looks identical! For most feminine nouns ending in -a (like soba), the genitive plural ending is also -a. In spoken Croatian, this final -a is drawn out longer to show it is plural, but in writing, you rely on context clues like the number petnaest to know it means "rooms".

Where does the -eva ending in ključeva come from?

The singular noun is ključ (key). Many short, one-syllable masculine nouns add an extra piece, either -ov- or -ev-, to make their plural forms. Because ključ ends in the "soft" consonant č, it uses -ev-. Finally, we add the standard genitive plural ending -a, giving us ključeva.

Do the numbers petnaest and šesnaest change their endings to match the nouns they count?

No. Unlike the numbers 1 through 4, numbers from 5 onwards are locked in place. They never change their spelling regardless of the gender or case of the noun they are counting.