Learn Croatian: Tko je ovaj čovjek?

The word for "this" has to match the gender of the noun it describes. Because čovjek (man) is a masculine noun, we use the masculine form ovaj. If we were asking about a woman (žena), we would use the feminine form ova, like in Tko je ova žena?

Why does je come right after Tko?

Je (is) is a short verb form, also called an enclitic. In Croatian, these short words are "second-position words" and usually have to be placed right after the first word in the sentence. So, it tucks in right behind Tko.

Can čovjek also mean "human" or "person"?

Yes! While it often translates to "man" (as an adult male), čovjek is also the general Croatian word for a "human being" or "person".

Are there different forms of tko depending on who I am asking about?

No, tko is used for everyone, exactly like "who" in English. It does not change based on the gender or the number of people you are asking about.