A little boy is swinging on a swing set.
The little boy is on a swing.
A little boy and the little boy could mean the same thing.  However, "a" is more specific.
exists e1 s1 x1 x2 x3.(on(e1,x2) & n_set(x2) & of(x2,x3) & n_swing(x3) & Actor(e1,x1) & v_swing(e1) & n_boy(x1) & a_little(s1) & Theme(s1,x1))
exists p1 s1 x1.(n_boy(x1) & a_little(s1) & Theme(s1,x1) & exists x2.(on(x1,x2) & n_swing(x2)))