Complex sentence: The girls plan to play a dirty trick on the boys.When the boys find out, both sides make a deal, that who ever makes the best Halloween costume will get to boss the other team around for a whole month.
Split into simpler sentences: The girls plan to play a dirty trick on the boys. But when the boys find out, both sides make a deal, that who ever makes the best Halloween costume will get to boss the other team what to do for a whole month.