Complex sentence: Jacob and his companions realise that Miss Peregrine can't change back to human form and when they go back to the orphanage they find it is destroyed which leaves them deciding to track down the wights and figure out how to help Miss Peregrine.
Split into simpler sentences: Jacob and his companions realise that Miss Peregrine can't change back to human form. When they return to the orphanage they find it destroyed, leaving them having to track down the wights and figure out how to help Miss Peregrine.