Complex sentence: Bill reveals to Sam that when Sam was a baby, he left Erica and she waited six months for him and when he came back, she accepted him and he promised that if she ever left him, he'd give her a second chance.
Split into simpler sentences: Bill reveals to Sam that when she was a baby, he left Erica and she waited six months for him. He promised that if she ever left him, he'd give her a second chance.