A person wearing a helmet begins to ride a skateboard down a street hill.
The surface of the street is sticky mud.
A person and a street are two different types of subjects.  Also, one is performing an action while the other is simply in a specified state.
exists e1 e2 p1 x1 x2 x3 x4 x5.(down(e2,x4) & n_hill(x4) & of(x4,x5) & n_street(x5) & Topic(e2,p1) & Actor(e2,x1) & v_begin(e2) & exists e3 x6.(Theme(e3,x6) & Actor(e3,x1) & v_ride(e3) & n_skateboard(x6)) & Theme(e1,x3) & Actor(e1,x2) & v_wear(e1) & n_helmet(x3) & (x1 = x2) & n_person(x1))
exists p1 x1 x2.(n_street(x2) & n_surface(x1) & exists s1 x3.((x1 = x3) & n_mud(x3) & a_sticky(s1) & Theme(s1,x3)) & of(x1,x2))