A surfer is on a wave.
A person surfing.
A surfer is a person and being on a wave is the action of surfing.
exists p1 x1.(exists x2.(on(x1,x2) & n_wave(x2)) & n_surfer(x1))
exists x1 x2.(a_topic(x1) & Time(x1,x2) & n_surfing(x2) & n_person(x1))