"Where are the stairs?"
Translation:Di mana tangganya?
My opinion is (I am also just student, so not sure):
"Di mana tangganya?" - "where are the stairs?" - they are speaking about specific stairs which are supposed to be there or which used to be there
"Dia jatuh dari tanga" - "He fell down from the stairs" doesnt matter which stairs he fell down. The result is his leg is broken and we just speak about the cause. I think this is what they mean in the lecture. Someone just informs another person about what what happened, doesnt matter from which stairs ("Oh, what happend to him?"..."He fell down the stairs"
"Dia jatuh dari tangganya" - He fell down from these/those stairs. We would say if we were standing right at the place where it happen and wanted to point on those specifis stairs