A surfer rides a green wave.
The surfer was riding a green wave.
"Was riding a green wave" is a past-tense rephrasing of "rides a green wave".
exists e1 s1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_wave(x2) & a_green(s1) & Theme(s1,x2) & n_surfer(x1))
exists e1 s1 x1 x2.(n_surfer(x1) & Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_wave(x2) & a_green(s1) & Theme(s1,x2))