Shouldn't a 2nd. Sg. form end in -sz? Even if "ön" is meant, wouldn't "Where does he/she swim" still be correct?
When a verb ends in one of s sz z dz, it would be difficult to pronounce an sz after that, so el ol is used instead -- see http://www.hungarianreference.com/Verbs/Verbs-Indefinite-Conjugation.aspx#s .
Thanks! Could you maybe put that explanation in the Tips & Notes section? Or is it going to be explained in detail later?
'Úszol' is correct. "Where does he/she swim" is also correct, but in this case, you should use 3rd Singular form, which is 'úszik'.