"I often hear rivers."
Translation:Gyakran hallok folyókat.
It is not wrong. "It is rivers that I hear often".
Usually I hear lakes, but often I hear rivers. - Gyakran folyókat hallok.
It is a little bit weird with "often", it would work better with "sometimes".
Often I hear rivers, other times I hear seas. - Gyakran folyókat hallok, máskor tengereket.
Anyway, "gyakran" refers to what comes after it.