Complex sentence: At the beginning of the movie Casper (Jeremy Foley) is on a train full of ghosts, with no idea of where he is and seems to be completely unaware that he himself is a ghost.
Split into simpler sentences: At the beginning of the movie Casper (Jeremy Foley) is on a train full of ghosts, with no idea of where he is. And seems to be completely unaware that he himself is a ghost.