Learn Croatian: Kupit ću devetnaest čokolada i dvadeset kolača. - I will buy nineteen chocolates and twenty cakes.

Because of the numbers! In Croatian, numbers from 5 and up (like devetnaest and dvadeset) require the noun to be in the genitive plural form. For both masculine nouns (like kolač) and feminine nouns (like čokolada), the genitive plural usually ends in "-a".

Wait, isn't čokolada just the basic singular word for "chocolate"? How can we tell it is plural?

You have a good eye! The dictionary form (nominative singular) for one chocolate is indeed čokolada. Coincidentally, the genitive plural form for most feminine nouns also ends in "-a". You can tell it means multiple chocolates here purely from the context of the number devetnaest (nineteen) coming right before it.

Why is the verb written as kupit instead of the full dictionary form kupiti?

This is a spelling rule for the future tense. When the "I will" helper word (ću) comes right after a main verb ending in "-ti", we drop the final "i" from the main verb. So, kupiti (to buy) becomes kupit ću (I will buy).

How are larger numbers like devetnaest and dvadeset formed?