Learn Croatian: Trebam stol za dvije osobe. - I need a table for two people.

In Croatian, the number two changes depending on the gender of the noun it counts. Because osoba (person) is a feminine noun, we use the feminine form dvije. If we were counting masculine or neuter things, we would use dva.

Why is the word for people osobe instead of the dictionary form osoba?

This is the special rule for the numbers 2, 3, and 4! When you use these numbers, the noun after them takes the genitive singular form. The genitive singular of the feminine noun osoba is osobe.

The preposition za (for) usually takes the accusative case, so why are we using a genitive singular rule here?

Great question! The preposition za does require the accusative case, and the number dvije is technically in the accusative here. However, the special rule for numbers 2, 3, and 4 forces the noun attached to them to take the genitive singular form anyway.

Why doesn't the sentence say Ja trebam for 'I need'?

In Croatian, you usually drop subject pronouns like ja (I) because the verb ending already tells you who is doing the action. The -m at the end of Trebam clearly shows it means 'I need'.