A man in a blue cap is playing a guitar while a young girl sits on a stool across from him and listens.
The man knows the girls.
Although a girl listens, it does not imply he knows the girl.  There is only one girl, not girls.
exists e1 p1 s1 x1 x2 x3.(exists e2 s2 s3 x4 x5 x6 x7.(from(e2,x6) & subset_of(x7,x6) & n_listen(x7) & subset_of(x1,x6) & a_across(s3) & Manner(e2,s3) & on(e2,x5) & n_stool(x5) & Actor(e2,x4) & v_sit(e2) & n_girl(x4) & a_young(s2) & Theme(s2,x4)) & n_male(x1) & while(e1,p1) & Theme(e1,x3) & Actor(e1,x1) & v_play(e1) & n_guitar(x3) & in(x1,x2) & n_cap(x2) & a_blue(s1) & Theme(s1,x2) & n_man(x1))
exists e1 x1 x2.(n_girl(x2) & n_man(x1) & Theme(e1,x2) & Actor(e1,x1) & v_know(e1))