A guy on a snowboard grinding.
A snowboarder performs in a competition.
You don't have to be in a competition to grind on a snowboard.
exists e1 x1 x2 x3.(a_topic(x1) & on(x1,x2) & Actor(e1,x3) & v_grind(e1) & (x2 = x3) & n_snowboard(x2) & n_guy(x1))
exists e1 x1 x2.(in(e1,x2) & n_competition(x2) & Actor(e1,x1) & v_perform(e1) & n_snowboarder(x1))