419.4. Replacement or prevention effects must exist before the appropriate event occurs-they can’t “go back in time” and change something that’s already happened. Usually spells and abilities that generate these effects are played in response to whatever would produce the event and thus resolve before that event would occur.

Example: A player can play a regeneration ability in response to a spell that would destroy a creature he or she controls.

419.5. If an event is prevented or replaced, it never happens. A modified event occurs instead, which may in turn trigger abilities. Note that the modified event may contain instructions that can’t be carried out, in which case the player simply ignores the impossible instruction.

419.5a If a source would deal 0 damage, it does not deal damage at all. That means abilities that trigger on damage being dealt won’t trigger. It also means that replacement effects that increase damage dealt have no event to replace, so they have no effect.

419.5b Some abilities read, “you may [X]. If you do, [Y].” An “if you do” clause that follows a “you may [X]” clause refers to choosing to do the event X, regardless of what events actually occur as a result of that decision. If X is replaced entirely or in part by a different event, the “if you do” clause refers to the event that replaced X.

419.6. Replacement Effects

419.6a A replacement effect doesn’t invoke itself repeatedly and gets only one opportunity for each event.

Example: A player controls two permanents, each with an ability that reads “If a creature you control would deal damage to a creature or player, it deals double that damage to that creature or player instead.” A creature that normally deals 2 damage will deal 8 damage-not just 4, and not an infinite amount.

419.6b Regeneration is a destruction-replacement effect. The word “instead” doesn’t appear on the card but is implicit in the definition of regeneration. “Regenerate [permanent]” means “The next time [permanent] would be destroyed this turn, instead remove all damage from it, tap it, and (if it’s in combat) remove it from combat.” Abilities that trigger from damage being dealt still trigger even if the permanent regenerates.

419.6c Some effects replace damage dealt to one creature or player with the same damage dealt to another creature or player; such effects are called “redirection” effects. If either creature is no longer in play or is no longer a creature when the damage would be redirected, the effect does nothing.

419.6d Some spells and abilities replace part or all of their own effect(s) when they resolve. Such effects are called self-replacement effects. When applying replacement effects to an event, apply self-replacement effects first, then apply other replacement effects.

419.6e Skipping an action, step, phase, or turn is a replacement effect. “Skip [something]” is the same as “Instead of doing [something], do nothing.” Once a step, phase, or turn has started, it can no longer be skipped-any skip effects will wait until the next occurrence.

419.6f Anything scheduled for a skipped step, phase, or turn won’t happen. Anything scheduled for the “next” occurrence of something waits for the first occurrence that isn’t skipped. If two effects each cause a player to skip his or her next occurrence, that player must skip the next two; one effect will be satisfied in skipping the first occurrence, while the other will remain until another occurrence can be skipped.

419.6g Some replacement effects say “instead choose one -.” Such effects are called modal replacement effects. The mode is chosen as the replacement effect is applied. If a modal replacement effect would apply to multiple events, a different mode may be chosen for each event. A modal replacement effect doesn’t invoke itself repeatedly, regardless of which mode was chosen. You may not choose modes that are impossible.

419.6h Some effects replace card draws. These effects are applied even if no cards could be drawn because there are no cards in the affected player's library. If an effect replaces a draw within a sequence of card draws, all actions required by the replacement are completed, if possible, before resuming the sequence. If an effect would have a player both draw a card and perform an additional action on that card, and the draw is replaced, the additional action is not performed on any cards that are drawn as a result of that replacement effect.

419.7. Prevention Effects

419.7a Prevention effects usually apply to damage that would be dealt.

419.7b Some prevention effects refer to a specific amount of damage-for example, “Prevent the next 3 damage that would be dealt to target creature or player this turn.” These work like shields. Each 1 damage that would be dealt to the “shielded” creature or player is prevented. Preventing 1 damage reduces the remaining shield by 1. If damage would be dealt to the shielded creature or player by two or more sources at the same time, the player or the controller of the creature chooses which damage the shield prevents first. Once the shield has been reduced to 0, any remaining damage is dealt normally. Such effects count only the amount of damage; the number of events or sources dealing it doesn’t matter.

419.7c Some prevention effects prevent the next N damage that would be dealt to each of a number of untargeted creatures. Such an effect creates a prevention shield for each applicable creature when the spell or ability that generates that effect resolves.

Example: Wojek Apothecary has an ability that says “{T}: Prevent the next 1 damage that would be dealt to target creature and each other creature that shares a color with it this turn.” When the ability resolves, it gives the target creature and each other creature in play that shares a color with it at that time a shield preventing the next 1 damage that would be dealt to it. Changing creatures’ colors after the ability resolves doesn’t add or remove shields, and creatures that come into play later in the turn don’t get the shield.

419.8. Sources of Damage

419.8a Some effects apply to damage from a source-for example, “The next time a red source of your choice would deal damage to you this turn, prevent that damage.” If an effect requires a player to choose a source, he or she may choose a permanent, a spell on the stack (including an artifact, creature, or enchantment spell), or any card or permanent referred to by an object on the stack (including a creature that assigned combat damage on the stack, even if the creature is no longer in play or is no longer a creature). The source is chosen when the effect is created. If the player chooses a permanent, the prevention will apply to the next damage from that permanent, regardless of whether it’s from one of that permanent’s abilities or combat damage dealt by it. If the player chooses an artifact, creature, or enchantment spell, the prevention will apply to any damage from that spell and from the permanent that it becomes when it resolves.

419.8b Some effects from spells and abilities prevent or replace damage from sources with certain properties, such as a creature or a source of a particular color. When the source would deal damage, the “shield” rechecks the source’s properties. If the properties no longer match, the damage isn’t prevented or replaced. If for any reason the shield prevents no damage or replaces no damage, the shield isn’t used up.