409.1i Once the steps described in 409.1a-409.1h are completed, the spell or ability becomes played. Any abilities that trigger on a spell or ability being played or put onto the stack trigger at this time. If the spell or ability’s controller had priority before playing it, he or she gets priority.
409.2. Some spells and abilities specify that one of their controller’s opponents does something the controller would normally do while it’s being played, such as choose a mode, choose targets, or choose how the spell or ability will affect its targets. In these cases, the opponent does so when the spell or ability’s controller normally would do so.
409.2a If there is more than one opponent who could make such a choice, the spell or ability’s controller decides which of those opponents will make the choice.
409.2b If the spell or ability instructs its controller and another player to do something at the same time as the spell or ability is being played, the spell’s controller goes first, then the other player. This is an exception to rule 103.4.
409.3. Playing a spell or ability that alters costs won’t do anything to spells and abilities that are already on the stack.
409.4. A player can’t begin to play a spell or activated ability that’s prohibited from being played by an effect.
410.1. Because they aren’t played, triggered abilities can trigger even when it isn’t legal to play spells and abilities, and effects that prevent abilities from being played don’t affect them.
410.2. Whenever a game event or game state matches a triggered ability’s trigger event, that ability triggers. When a phase or step begins, all abilities that trigger “at the beginning of” that phase or step trigger. The ability is controlled by the player who controlled its source at the time it triggered. It has the text of the ability that created it, and no other characteristics. The ability doesn’t do anything when it triggers, but it’s automatically put on the stack by its controller as soon as a player would receive priority.
410.3. If multiple abilities have triggered since the last time a player received priority, each player, in APNAP order, puts triggered abilities he or she controls on the stack in any order he or she chooses. (See rule 103.4.) Then players once again check for and resolve state-based effects until none are generated, then abilities that triggered during this process go on the stack. This process repeats until no new state-based effects are generated and no abilities trigger. Then the appropriate player gets priority.
410.4. When a triggered ability goes on the stack, the controller of the ability makes any choices that would be required while playing an activated ability, following the same procedure (see rule 409, “Playing Spells and Activated Abilities”). If no legal choice can be made (or if a rule or a continuous effect otherwise makes the ability illegal), the ability is simply removed from the stack.
410.5. Some triggered abilities’ effects are optional (they contain “may,” as in “At the beginning of your upkeep, you may draw a card”). These abilities go on the stack when they trigger, regardless of whether their controller intends to exercise the ability’s option or not. The choice is made when the ability resolves. Likewise, triggered abilities that have an effect “unless” something is true or a player chooses to do something will go on the stack normally; the “unless” part of the ability is dealt with when the ability resolves.
410.6. An ability triggers only once each time its trigger event occurs. However, it can trigger repeatedly if one event contains multiple occurrences. See also rule 410.9.
Example: A permanent has an ability whose trigger condition reads, “Whenever a land is put into a graveyard from play, . . . .” If someone plays a spell that destroys all lands, the ability will trigger once for each land put into the graveyard during the spell’s resolution.
410.7. An ability triggers only if its trigger event actually occurs. An event that’s prevented or replaced won’t trigger anything.
Example: An ability that triggers on damage being dealt won’t trigger if all the damage is prevented.
410.8. Triggered abilities with a condition directly following the trigger event (for example, “When/Whenever/At [trigger], if [condition], [effect]”), check for the condition to be true as part of the trigger event; if it isn’t, the ability doesn’t trigger. The ability checks the condition again on resolution. If it’s not satisfied, the ability does nothing. Note that this mirrors the check for legal targets. Note that this rule doesn’t apply to any triggered ability with an “if” condition elsewhere within its text. This rule is referred to as the “intervening ‘if’ clause” rule.
410.9. Some abilities trigger when creatures block or are blocked in combat. (See rules 306-311 and rule 500, “Legal Attacks and Blocks.”) They may trigger once or repeatedly, depending on the wording of the ability.
410.9a An ability that reads “Whenever [this creature] blocks, . . .” triggers only once each combat for that creature, even if it blocks multiple creatures. It triggers only if the creature is declared as a blocker.
410.9b An ability that reads “Whenever [this creature] blocks a creature, . . .” triggers once for each attacking creature the creature with the ability blocks. It triggers only if the creature is declared as a blocker.
410.9c An ability that reads “Whenever [this creature] becomes blocked, . . .” triggers only once each combat for that creature, even if it’s blocked by multiple creatures. It will also trigger if an effect causes a creature to block the attacking creature, but only if it hadn’t already been blocked that combat. It will trigger if the creature becomes blocked by an effect rather than a creature.
410.9d An ability that reads “Whenever a creature blocks [this creature], . . .” triggers once for each creature that blocks the named creature. It will also trigger if an effect causes a creature to block the attacking creature, even if it had already been blocked that combat. It won’t trigger if the creature becomes blocked by an effect rather than a creature.
410.9e If an ability triggers when a creature blocks or becomes blocked by a particular number of creatures, the ability triggers if the creature blocks or is blocked by that many creatures when the attack or block declaration is made. Effects that add or remove blockers can cause such abilities to trigger. This also applies to abilities that trigger on a creature blocking or being blocked by at least a certain number of creatures.
410.10. Trigger events that involve objects changing zones are called “zone-change triggers.” Many abilities with zone-change triggers attempt to do something to that object after it changes zones. During resolution, these abilities look for the object in the zone that it moved to. If the object is unable to be found in the zone it went to, the part of the ability attempting to do something to the object will fail to do anything. The ability could be unable to find the object because the object never entered the specified zone, because it left the zone before the ability resolved, or because it is in a zone that is hidden from a player, such as a library or an opponent’s hand. (This rule applies even if the object leaves the zone and returns again before the ability resolves.) The most common types of zone-change triggers are comes-into-play triggers and leaves-play triggers.