why ? (merci de me repondre !)

308. Declare Attackers Step

308.1. As the declare attackers step begins, the active player declares attackers (this game action doesn’t use the stack). Then any abilities that triggered on attackers being declared go on the stack. (See rule 410, “Handling Triggered Abilities.”) Then the active player gets priority and players may play spells and abilities.

308.2. To declare attackers, the active player follows the steps below, in order. If at any point during the declaration of attackers, the active player is unable to comply with any of the steps listed below, the declaration was illegal; the game returns to the moment before the declaration (see rule 422, “Handling Illegal Actions,” and rule 500, “Legal Attacks and Blocks”).

308.2a The active player either chooses to not attack or chooses one or more creatures he or she controls, and then determines whether this set of creatures could attack. Only creatures can attack, and the following creatures can’t attack: tapped creatures (even those that can attack without tapping), creatures with creature type Wall, and creatures the active player didn’t control continuously since the beginning of the turn (except those with haste). Other effects may also affect whether or not a set of creatures could attack. (See rule 500, “Legal Attacks and Blocks.”)

308.2b If any of the chosen creatures have banding or a bands with other ability, the active player announces which creatures, if any, are banded with which. (See rule 502.10, “Banding.”)

308.2c The active player taps the chosen creatures. Tapping a creature when it’s declared as an attacker isn’t a cost; attacking simply causes creatures to become tapped.

308.2d If any of the creatures require paying costs to attack, the active player determines the total cost to attack. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. Once the total cost is determined, it becomes “locked in.” If effects would change the total cost after this time, ignore this change.

308.2e If any of the costs require mana, the active player then has a chance to play mana abilities (see rule 411, “Playing Mana Abilities”).

308.2f Once the player has enough mana in his or her mana pool, he or she pays all costs in any order. Partial payments are not allowed.

308.2g Each chosen creature becomes an attacking creature if all costs have been paid, but only if it’s still controlled by the active player. It remains an attacking creature until it’s removed from combat or the combat phase ends, whichever comes first. See rule 306.2.

308.3. Abilities that trigger on a creature attacking trigger only at the point the creature starts to attack. They will not trigger if a creature attacks and the characteristics of that creature are then changed to match the ability’s trigger condition.
Example: A permanent has the ability “Whenever a green creature attacks, destroy that creature at end of combat.” If a blue creature attacks and is later turned green, the ability will not trigger.

308.4. If no creatures are declared as attackers, finish the declare attackers step, but skip the declare blockers and combat damage steps.

404. Triggered Abilities

404.1. A triggered ability begins with the word “when,” “whenever,” or “at.” The phrase containing one of these words is the trigger condition, which defines the trigger event.

404.2. Triggered abilities aren’t played. Instead, a triggered ability automatically “triggers” each time its trigger event occurs. Once an ability has triggered, it goes on the stack the next time a player would receive priority. See rule 408.1, “Timing, Priority, and the Stack,” and rule 410, “Handling Triggered Abilities.”

404.3. A triggered ability may read “When/Whenever/At . . . , if [condition], [effect].” The ability checks for the stated condition to be true when the trigger event occurs. If it is, the ability triggers. On resolution, the ability rechecks the condition. If the condition isn’t true at either of those times, the ability does nothing. This rule is referred to as the “intervening ‘if’ clause” rule. Note that the word “if” has only its normal English meaning anywhere else in the text of a card; this rule only applies to an “if” that immediately follows a trigger condition.

404.4. An effect may create a delayed triggered ability that can do something at a later time. A delayed triggered ability will contain “when,” “whenever,” or “at,” although that word won’t usually begin the ability.

404.4a Delayed triggered abilities come from spells or other abilities that create them on resolution. That means a delayed triggered ability won’t trigger until it has actually been created, even if its trigger event occurred just beforehand. Other events that happen earlier may make the trigger event impossible.
Example: Part of an effect reads “When this creature leaves play,” but the creature in question leaves play before the spell or ability creating the effect resolves. In this case, the delayed ability never triggers. As another example, if an effect reads “When this creature becomes untapped” and the named creature becomes untapped before the effect resolves, the ability waits for the next time that creature untaps.

404.4b A delayed triggered ability will trigger only once—the next time its trigger event occurs—unless it has a stated duration, such as “this turn.”

404.4c A delayed triggered ability that refers to a particular object still affects it even if the object changes characteristics.
Example: An ability reading, “At end of turn, destroy that creature” will destroy the permanent even if it’s no longer a creature during the end of turn step.

404.4d A delayed triggered ability that refers to a particular permanent will fail if the permanent leaves play (even if it returns again before the specified time). Similarly, abilities that create a one-shot effect that applies to an object in a particular zone will fail if the object leaves that zone.
Example: An ability reading, “At end of turn, remove this creature from the game” won’t do anything if the creature leaves play before the end of turn step.