Complex sentence: Within these three days, Ariel must receive the "kiss of true love" from Eric; otherwise, she will transform back into a mermaid and belong to Ursula.
Split into simpler sentences: Within these three days, Ariel must receive the "kiss of true love" from Eric. If Ariel gets Eric to kiss her, she will remain a human permanently, otherwise, she will transform back into a mermaid and belong to Ursula.