Could someone explain why folyó requires the ´n´ at the end? I thought that ending would suggest ´jumping in´. Or, if the river is considered the direct object receiving the action, I could see it becoming folyót with the ending to show it was the object. I am officially confused (but enjoying learning such a complex language).
The n is not replacing ban-ben (in) but -on (on). When I jump across it, I have the river underneath me, so I am ON it.
Átugrom a folyón: I jump across the river (from shore to shore)
Beugrom a folyóba: I jump into the river (getting wet)
Ugrom a folyóban: I am jumping in the river (I was already in it when I started my jump)
It's an -ikes ige. The form is not different in definite/indefinite with I. But átugrik can be transitive, although simply 'ugrik' can't. the prefix modifies the meaning.
Átugrasz a folyón - átugrod a folyót.