A man without a helmet is riding a red bike.
A man is riding a mountain bike on the street.
You can ride a bike elsewhere than on the street, and you can ride other kinds of bikes than mountain bikes.
exists e1 s1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_bike(x2) & a_red(s1) & Theme(s1,x2) & -exists x3.(with(x1,x3) & n_helmet(x3)) & n_man(x1))
exists e1 x1 x2 x3 x4.(n_street(x4) & on(e1,x4) & Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_bike(x2) & of(x2,x3) & n_mountain(x3) & n_man(x1))