A blond girl sits on a white porch railing.
A girl is sitting on the railing.
Sitting is a present participle of sits and she's on the railing.
exists e1 s1 s2 x1 x2 x3.(on(e1,x2) & n_railing(x2) & of(x2,x3) & n_porch(x3) & a_white(s2) & Theme(s2,x2) & Actor(e1,x1) & v_sit(e1) & n_girl(x1) & a_blond(s1) & Theme(s1,x1))
exists e1 x1 x2.(n_railing(x2) & on(e1,x2) & Actor(e1,x1) & v_sit(e1) & n_girl(x1))