A British guard marches while on duty.
A British guard marches  on the street as Kate Middleton approaches
A British guard could be marching anywhere, not only on the street.  A British guard could be marching regardless of whether Kate Middleton approaches.
(x)
exists s1 x1 x2 x3 x4 x5 x6.(n_street(x3) & a_topic(x1) & as(x1,x4) & n_approach(x4) & (x4 = x6) & ne_per_middleton(x6) & (x4 = x5) & ne_per_kate(x5) & on(x1,x3) & n_march(x1) & of(x1,x2) & n_guard(x2) & a_british(s1) & Theme(s1,x1))