The band were playing in the stage.
The band played on the stage.
The band "Were playing" means the same as "played on" the stage, as both refer to the past.  (the first is called  past imperfect)
exists e1 x1 x2.(n_stage(x2) & n_band(x1) & in(e1,x2) & Actor(e1,x1) & v_play(e1))
exists e1 x1 x2.(n_stage(x2) & n_band(x1) & on(e1,x2) & Actor(e1,x1) & v_play(e1))