I could be wrong, but the conditional clause is past tense, as in if you already cast it, you may cast if for free
Casting a spell is a complex nine-step process that goes from 601.2a to 601.2i in the comprehensive rules. Choosing an alternate casting mode occurs in the second step of the process, 601.2b, while the actual determination of a spell's cost, the activation of mana abilities, and the payment of costs occur in the sixth, seventh, and eighth steps, 601.2f, g, and h, respectively. There is plenty of room for the rules to see what's going on here, so much so that the casting rules even allow for really weird stuff like Garruk's Horde letting its controller cast Zoetic Cavern for its morph cost off of the top of the deck.
I don' think you can cast once upon a time for free unless you "can" pay for it.
No,the only restriction is that it's the first spell you've cast in the game. The game can check permanents under a player's control, like with Invigorate, but whether a player "can" cast a spell for its printed mana cost, as you put it, is out of the game's scope, anyways.
Why turn 2 when you can slam it turn 1? I would wager that more than a few combo decks will use this to keep otherwise unusable hands.
Casting a spell is a complex nine-step process that goes from 601.2a to 601.2i in the comprehensive rules. Choosing an alternate casting mode occurs in the second step of the process, 601.2b, while the actual determination of a spell's cost, the activation of mana abilities, and the payment of costs occur in the sixth, seventh, and eighth steps, 601.2f, g, and h, respectively. There is plenty of room for the rules to see what's going on here, so much so that the casting rules even allow for really weird stuff like Garruk's Horde letting its controller cast Zoetic Cavern for its morph cost off of the top of the deck.
No,the only restriction is that it's the first spell you've cast in the game. The game can check permanents under a player's control, like with Invigorate, but whether a player "can" cast a spell for its printed mana cost, as you put it, is out of the game's scope, anyways.