Learn Croatian: Tko je on?

je (is) is a short, unstressed word called a clitic. In Croatian, clitics must go in the second position of a sentence. Since Tko (who) is the first word, je has to step right in line behind it, pushing on (he) to the end.

Since Croatian drops pronouns a lot, can I just say 'Tko je?'

You can, but it changes the meaning slightly. If you just ask 'Tko je?', it usually translates to 'Who is it?' (for example, when someone knocks on a door). By keeping the pronoun on, you specifically ask 'Who is he?'.

How would I change this sentence to ask 'Who is she?'

You just swap the masculine pronoun on (he) for the feminine pronoun ona (she), giving you 'Tko je ona?'. The verb je works for he, she, and it, so it does not need to change.

What is the base verb for je?

je is the short form of the verb biti (to be) for he, she, and it. It belongs to the same group as sam (I am) and si (you are) that you are learning in this lesson.