The stack.

[Taysby]

What exactly can you do something "in response" to?
Can you respond to your Opponent declaring something?  For ex.  My opponent declares he is attacking with a creature.  Can I tap it in response to him declaring it as an attacker, so it can't attack because its tapped?

Also, another question.  I have out {llanowar elves} and attack.  It gets blocked by {drogskul reaver}. Does the reaver hit the elf twice?

[Kaleo42]

You can respond to phase changes, triggering or activating abilities (ones with "whenever" or ":" respectively), attacks, and blockers (which is where you have to respond if you want things to happen before damage).

If you wait till they declare their attackers it's too late to tap then to stop them since they will still have been declared attackers. You can respond to "moving to combat" which if they dont say before trying to attack you are welcome to back them up and say "in RESPONSE to you moving to combat, ill tap X and Y."

As for the double strike question, the first strike will (i assume) kill the elf leaving no target for the second strike unless it has trample and therefore would be ableto reassign extra damage to the player. So assuming no outside effects, they would kill your wlf, gain 3, and draw a card, nothing more.

[Mentonin]

1- you can respond to anything that uses the stack. You can also respond to phases and turns changes. Attacking, however, doesn't use the stack. If you want to tap something to stop it from attacking, here is when you can do it:
   •On your opponent's first main phase, whenever you get priority (after he casts something, or when he declares end of main phase)
   •On the beginning of combat phase, whenever you get priority (after he casts something, or when he declares end of beginning of combat)

If you wait for the declare attackers phase, he will declare and tap attackers, and this doesn't use the stack, which means that any tapping you do thereafter doesn't cancel the attack because it is already attacking


2- the drogskol has double strike, which means he deals damage both on the first strike damage phase and the normal damage phase. In the first strike damage phase, unless it got trample somehow, he will deal all damage to the blocker and, in the case of the elf, kill him. Then, on the normal combat phase, it will still be "blocked", and try to deal all his damage to the blocker(s) (unless he has trample, in which case all exceeding damage is dealt to the defending player). If all blocking creatures died already (as in the example provided), the damage on the normal damage phase will have no target, and "fizzle". Meaning he will deal only the first strike damage.


I hope I wa clear