A construction worker is fixing a large machine.
a man on a tractor works a field
A worker fixing a machine isn't usually on it.
exists e1 s1 x1 x2 x3.(Theme(e1,x3) & Actor(e1,x1) & v_fix(e1) & n_machine(x3) & a_large(s1) & Theme(s1,x3) & n_worker(x1) & of(x1,x2) & n_construction(x2))
exists e1 x1 x2 x3.(Theme(e1,x3) & Actor(e1,x1) & v_work(e1) & n_field(x3) & on(x1,x2) & n_tractor(x2) & n_man(x1))