A boy on a skateboard is at the top of a ramp.
The boy is on the skateboard
"On a skateboard" and "on the skateboard" mean the same thing. Both refer to a single skateboard which someone is on.
exists p1 x1 x2 x3.(n_top(x3) & exists x4.(at(x1,x3) & of(x3,x4) & n_ramp(x4)) & on(x1,x2) & n_skateboard(x2) & n_boy(x1))
exists p1 x1 x2.(n_skateboard(x2) & n_boy(x1) & on(x1,x2))