Players
playTo play a land means to put it onto the battlefield from the zone it’s in (usually the hand). A player may play a land if they have priority, it’s the main phase of their turn, the stack is empty, and they haven’t played a land this turn. Playing a land is a special action (see rule 116), so it doesn’t use the stack; it simply happens. Putting a land onto the battlefield as the result of a spell or ability isn’t the same as playing a land. See rule 305, “Lands.”
To play a card means to play that card as a land or to cast that card as a spell, whichever is appropriate.
Some effects instruct a player to “play” with a certain aspect of the game changed, such as “Play with the top card of your library revealed.” “Play” in this sense means to play the Magic game.
Previously, the action of casting a spell, or casting a card as a spell, was referred to on cards as “playing” that spell or that card. Cards that were printed with that text have received errata in the Oracle card reference so they now refer to “casting” that spell or that card.
Previously, the action of using an activated ability was referred to on cards as “playing” that ability. Cards that were printed with that text have received errata in the Oracle card reference so they now refer to “activating” that ability. (CR 701.18) a Magic subgame, starting at 1 life and using their libraries as their decks. For the rest of the main game, if a source would deal damage to a player who didn't win the subgame, it deals
doubleDoubling a creature’s power and/or toughness creates a continuous effect. This effect modifies that creature’s power and/or toughness but doesn’t set those characteristics to a specific value. See rule 613.4c.
To double a creature’s power, that creature gets +X/+0, where X is that creature’s power as the spell or ability that doubles its power resolves. Similarly, an effect that doubles a creature’s toughness gives it +0/+X, where X is that creature’s toughness. Doubling a creature’s power and toughness gives it +X/+Y, where X is its power and Y is its toughness.
If a creature’s power is less than 0 when it’s doubled, doubling that creature’s power instead means that the creature gets -X/-0, where X is the difference between 0 and its power. Similarly, if its toughness is less than 0 when doubled, it gets -0/-X. If one characteristic’s value is negative but the other isn’t when both are doubled, it gets -X/+Y or +X/-Y, as appropriate.
To double a player’s life total, the player gains or loses an amount of life such that their new life total is twice its current value.
To double the number of a kind of counters on a player or permanent, give that player or permanent as many of those counters as that player or permanent already has.
To double the amount of a type of mana in a player’s mana pool, that player adds an amount of mana of that type equal to the amount they already have.
To double an amount of damage a source would deal, that source instead deals twice that much damage. This is a replacement effect. (CR 701.10) that damage to that player instead.