a biker rides down the street.
A biker is riding
A biker riding down the street implies that the biker is riding.
exists e1 x1 x2.(n_street(x2) & down(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_biker(x1))
exists e1 x1.(Actor(e1,x1) & v_ride(e1) & n_biker(x1))