Complex sentence: When he and his wife Helen were first looking at the house, Jeffrey's son David came into the house and explains to Stanley that if his father didn't like him, he would never let them (he and Helen) move in.
Split into simpler sentences: When he and his wife Helen were first looking at the house, Jeffrey's son David came into the house. He explained to Stanley that if his father didn't like him, he would never let them (he and Helen) move in.