Bajo vs debajo
Many spanish speakers I know use debajo to mean under/underneath however duolingo simply uses bajo, which they use to mean "low". Is this simply regional differences? Most I know are from the Caribbean.
They are interchangeable. Both "bajo la mesa" and "debajo de la mesa" are correct. I guess it's mostly a regional difference, like you said. I've found "debajo" is usually easier for non-native speakers, because it helps to compartmentalize bajo = short/low, debajo = under/underneath, but it's really up to you.
Actually, where I live, we use "abajo" instead. As in "abajo de la mesa".