Learn Croatian: On se bavi ludošću. - He is dealing with madness.

The verb baviti se (to deal with, to be engaged in) always takes its object in the bare instrumental case, meaning without the preposition s. English speakers often want to add it because of the word 'with' in the translation, but you should just use the noun on its own in the instrumental.

What is the dictionary form of ludošću?

The nominative (dictionary) form is ludost (madness). Because it is a feminine noun ending in a consonant, it belongs to the feminine i-declension group.

How exactly does ludost turn into ludošću in the instrumental?

Feminine i-declension nouns often go through a sound change called jotation in the instrumental singular. When a noun ends in -st like ludost, the jotation rule turns that consonant cluster into šć before adding the ending -u. This gives us ludošću.

Could I just use ludosti instead of ludošću here?

Yes! For feminine i-declension nouns, the instrumental singular without jotation (ludosti) is also grammatically correct. However, the jotated form (ludošću) is very common, especially in standard or written Croatian, so it is important to recognize it.