Question: what is the difference between keldeo forms?
Answer: Keldeo has two interchangeable forms: Ordinary Form and Resolute Form. These forms are mostly cosmetic. The Resolute Form was introduced in Pokémon Black 2 and White 2. If Keldeo is taught Secret Sword in Pokémon Black 2 or White 2 or a Generation VI game, it will change into Resolute Form.