Complex sentence: Whereas the film adaptation was essentially a romantic drama, the novel, which includes themes of prostitution, rape, abortion and terminal illness, is significantly more grim and only some of the characters that appeared in the source novel appear in the film.
Split into simpler sentences: Whereas the film adaptation was essentially a romantic drama, the novel includes themes of prostitution, rape, abortion and terminal illness, and is significantly more grim. Many of the characters that appeared in the source novel do not appear in the film.