The boy has his mouth open
There is a boy.
A boy has his mouth open, therefore there is a boy.
exists e1 x1 x2 x3.(n_open(x2) & of(x2,x3) & n_mouth(x3) & of(x2,x1) & n_male(x1) & n_boy(x1) & Theme(e1,x2) & Actor(e1,x1) & v_have(e1))
exists p1 x1 x2.((x1 = x2) & n_boy(x2))