601.3 A player can begin to cast a spell only if a rule or effect allows that player to cast it and no rule or effect prohibits that player from casting it.
601.3a
If an effect prohibits a player from casting a spell with certain qualities, that player may consider any choices to be made during that spell’s proposal that may cause those qualities to change. If any such choices could cause that effect to no longer prohibit that player from casting that spell, the player may begin to cast the spell, ignoring the effect. Example: A player controls Void Winnower, which reads, in part, “Your opponents can’t cast spells with even mana values.” That player’s opponent may begin to cast Rolling Thunder, a card whose mana cost is {X}{R}{R}, because the chosen value of X may cause the spell’s mana value to become odd.
601.3b
If an effect allows a player to cast a spell with certain qualities as though it had flash, that player may consider any choices to be made during that spell’s proposal that may cause that spell’s qualities to change. If any such choices could cause that effect to apply, that player may begin to cast that spell as though it had flash. Example: An effect says that you may cast Aura spells as though they had flash, and you have a creature card with bestow in your hand. Because choosing the bestow ability’s alternative cost causes that spell to become an Aura spell, you may legally begin to cast that spell as though it had flash.
601.3c
If an effect allows a player to cast a spell as though it had flash only if an alternative or additional cost is paid, that player may begin to cast that spell as though it had flash.
601.3d
If a spell would have flash only if certain conditions are met, its controller may begin to cast that spell as though it had flash if those conditions are met.
601.3e
Some rules and effects state that an alternative set of characteristics or a subset of characteristics are considered to determine if a card or copy of a card is legal to cast. These alternative characteristics replace the object’s characteristics for this determination. Continuous effects that would apply to that object once it has those characteristics are also considered. Example: Garruk’s Horde says, in part, “You may cast creature spells from the top of your library.” If you control Garruk’s Horde and the top card of your library is a noncreature card with morph, you may cast it using its morph ability. Example: Melek, Izzet Paragon says, in part, “You may cast instant and sorcery spells from the top of your library.” If you control Melek, Izzet Paragon and the top card of your library is Giant Killer, an adventurer creature card whose Adventure is an instant named Chop Down, you may cast Chop Down but not Giant Killer. If instead you control Garruk’s Horde and the top card of your library is Giant Killer, you may cast Giant Killer but not Chop Down.
601.3f
Some effects allow a player to cast a spell with certain qualities from among face-down cards in exile. A player may begin to cast such a spell only if they can look at the face-down card in exile.