Learn Croatian: Jesu li sobe slobodne? - Are the rooms available?

Je li translates to "is it" and is used for singular subjects. Because we are asking about multiple rooms (sobe), we have to use the plural form "are they", which is jesu li. Jesu is the full, emphatic form of the verb "are" for "they".

How do we know sobe is plural?

The singular word for "room" is soba, which is a feminine noun ending in -a. To make feminine nouns ending in -a plural, you change the ending to -e. So, soba becomes sobe (rooms).

Why does the adjective slobodne end in -e?

Adjectives in Croatian must match the noun they describe. Since sobe is feminine and plural, the dictionary form of the adjective (slobodan) has to change to match. For feminine plural nouns, the adjective ending is -e, giving us slobodne.

Does slobodne mean "free" as in costing no money, or just "available"?

In this context, it means "available" or "unoccupied". While slobodan can mean "free" (like having freedom or free time), if you want to say something costs zero money, you must use a different word entirely: besplatan.