somebody in the air on a board
The person is on a skateboard.
A person can be in the air on a board without it being a skateboard.
exists x1 x2 x3.(n_air(x2) & a_topic(x1) & in(x1,x2) & on(x2,x3) & n_board(x3) & n_person(x1))
exists p1 x1.(n_person(x1) & exists x2.(on(x1,x2) & n_skateboard(x2)))