Return the Past

If a card with no mana cost gains flashback, it has no flashback cost. It can't be cast this way.

If a split card gains flashback, you pay only the cost of the half you're casting.

"Flashback [cost]" means "You may cast this card from your graveyard by paying [cost] rather than paying its mana cost" and "If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack."

If you cast an instant or sorcery with {X} in its mana cost this way, you still choose the value of X as part of casting the spell and pay that cost.

A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.

If a card is put into your graveyard during your turn, you can cast it with flashback if it's legal to do so before any other player can take any actions.