Learn Croatian: Trebam kilogram rajčica. - I need a kilogram of tomatoes.

We use the genitive case to show quantity, similar to the English word 'of'. Because you need a kilogram of tomatoes, the noun following the measurement word kilogram must take the genitive form.

rajčica is the singular word for tomato, so why does the translation say 'tomatoes'?

This is a tricky part of the genitive plural! For many regular feminine nouns that end in -a in their basic form, the genitive plural ending is also -a. So, rajčica can mean 'a tomato' (singular) or 'of tomatoes' (genitive plural). We rely on context, like buying a whole kilogram, to know it is plural here.

Where is the word for 'I' in this sentence?

Croatian frequently drops subject pronouns like ja (I) because the verb's ending already tells us who is doing the action. The -m at the end of trebam makes it perfectly clear that 'I' am the one needing.

What case is the word kilogram in here?

It is in the accusative case because it is the direct object of the verb, meaning it is the thing that you need. However, because kilogram is a masculine, non-living noun, its accusative form looks exactly the same as its basic dictionary form.