"Who is he?"
Translation:Kuka hän on?
As pieni_chilipalko wrote, the funny thing with Finnish is, if the words are in correct form, in many cases you can just slap the words into a sentence in a random order and eventually you would get understood.
However, some words require other words to be in specific places to convey the correct meaning, and by altering the word order and stress you can emphasize different ideas.
kuka hän on? - who is he?
kuka on hän? - who is he?
hän on kuka? - he is who?
hän kuka on?, on hän kuka?, on kuka hän? - (you would get your question across but in general case I would think you are either a non-native or a completely native lunatic).
Simplistically put, word order is fairly free (although in this case and in similar questions the preferred order is question word (kuka), pronoun (hän), verb (on)), it's the suffixes (case endings and such) which inform people of a particular word's purpose in a sentence, if it's unclear.
Please read the previous comments on the same subject. Another word order would get understood, but some of them would be considered strange. People would think you are either a poet, wierd or just not a native speaker. (To summarize the previous comments). In a beginners course, we need to learn the normal way of speaking.