Learn Croatian: Poljoprivrednik radi vani, a liječnik radi unutra. - The farmer works outside, while the doctor works inside.

In Croatian, a is used to contrast two parallel situations that don't contradict each other. Here, you are just comparing two different people doing different things in different places. If you used ali (but), it would imply some sort of conflict or unexpected twist, like the farmer working outside but hating the cold.

Poljoprivrednik is a very long word! Is there an easier way to remember it?

Yes, you can break it down into its parts! It comes from polje (field) and privreda (economy or agriculture). The ending -nik is a common suffix used for professions, just like the one you see in liječnik (doctor).

Why is there a comma before a? In English we wouldn't always use one before "and" or "while".

Croatian punctuation rules are very strict about this. You must always place a comma before the contrasting conjunctions a (while/and) and ali (but).

Do the words vani (outside) and unutra (inside) change ending depending on who is working?

No, they are adverbs of place, which means they never change their shape. They will always stay vani and unutra, regardless of the gender or number of the people you are talking about.