Learn Croatian: Ponedjeljkom učim hrvatski. - On Mondays I study Croatian.

It is in the instrumental case. In Croatian, putting a day of the week into the instrumental case without a preposition shows that an action happens regularly. So, ponedjeljkom means "on Mondays" or "every Monday".

I learned that "on Monday" is u ponedjeljak. What is the difference?

U ponedjeljak (using the preposition u + accusative case) refers to one specific Monday, like "I will see you on Monday." Using the instrumental ponedjeljkom means it is a recurring event happening every Monday.

Why isn't hrvatski capitalized like it is in English?

In Croatian, names of languages are written with a lowercase starting letter. You only capitalize them if they are the very first word in a sentence. However, names of countries and nationalities are capitalized, like Hrvatska for Croatia.

Can učim mean both "I study" and "I learn"?

Yes! The verb učiti is used for both "to study" (the process of trying to understand something) and "to learn". The context usually makes it clear which one is meant, though here both translations work perfectly well.