A surfer is riding a wave.
A man is on a wave.
Women surf too so the surfer isn't necessarily a man.
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_wave(x2) & n_surfer(x1))
exists p1 x1.(exists x2.(on(x1,x2) & n_wave(x2)) & n_man(x1))