Remember, on your opponent's turn they have priority. In short, that means that they get a chance to do things first (although you can respond as soon as they do). This is often an advantage. They must make decisions without knowing whether you intend to make a play during that phase. If you opt not to after they've passed, the step ends.
Your Opponent's Turn
When and why to initiate spells
1. Beginning Phase
Untap step: All of your opponent's tapped permanents untap. Nothing can be played during this step.
Upkeep step: The most significant tactical aspect of your opponent's upkeep step is that it is after they untap, but before they draw. It is also your first chance to make a play on your opponent's turn.
Generally, if your opponent is tapped out on your own turn and it's important that you resolve a spell, you should play it on your own turn. If they already have relevant permanents (generally lands) untapped, however, you might as well wait until their upkeep. That way if they respond then at least they'll have to tap permanents they could have used during their turn.
Opponent's-turn-related: It is the earliest phase in your opponent's turn. If you have a spell that specifically needs to be played during your opponent's turn to be useful, this is often the time to do it.Example: During my upkeep, you play Orim's Chant.
Mana-deprivation-related: Similarly, if you have an effect that taps an opponent's mana sources, this is usually the time to use them. Yes, they can use the mana for spells during their beginning phase, but they won't be able to use it for the non-instants.
Upkeep-effect-related: As with your own turn, sometimes it's important to navigate various upkeep effects.
Draw step: The most significant tactical aspect of your opponent's draw step is that it is after they've drawn, but before they're able to initiate non-instant spells. *
Hand-size-related: If a card's utility depends on your opponent's number of cards in hand, this may be a good time to play it.Example: During my draw step, you target me with Sudden Impact.
Example: I have no cards in hand, and then I draw for the turn. During my draw step, you target me with Funeral Charm's discard ability.
2. First Main Phase
The most significant tactical aspect of your opponent's first main phase is that it is before combat. They'll be positioning themselves for a successful combat step. You'll often have opportunities to respond to spells during this phase.
3. Combat Phase
Beginning of Combat step: The most significant tactical aspect of your opponent's beginning of combat step is that it is after they are able to play non-instants, but before they have declared their attackers.
Tap effects: This is the time to tap potential attackers.Sometimes this is done during the upkeep, or in response to card draw, in case your opponent draws a split second card (say, Krosan Grip for your Icy Manipulator), countermagic, or an untargetability effect, but in general it's best to wait until the last moment. This way, you have all the information on what they've done during their first main phase.
Declare Attackers step: Your opponent starts their declare attackers step by deciding which creatures are going to attack and tapping them. Then, each player gets a chance to play spells and abilities. The most significant tactical aspect of your opponent's declare attackers step is that it is after your opponent has committed to their attacks, but before you have committed to your blocks. This is a time to gather information.
Information-related: You can force your opponent to use a spell or ability before they know how you're going to block, or check to make sure a spell or ability of your own resolves the way you want it to before making a block.
Example: I'm at 2 life with a Grizzly Bears in play. You attack me with Raging Goblin and Craw Wurm. I Dark Banishing the Craw Wurm before blockers. If you have a counterspell to save the Wurm, I will be forced to blockit, but at least I won't be dead. If you don't, then I get to safely block and kill the Raging Goblin.Example: You attack me with two Hill Giants, and you have an Angelic Page untapped. I have a Craw Wurm. I Shock your Angelic Page now. Whichever Hill Giant you pump, I can leave unblocked and take 4 damage. Whichever you don't, I can kill with Craw Wurm.
Declare Blockers step: You start this step by declaring which of your creatures will block which attacking creatures. Then, each player has the chance to play spells or abilities. After you have declared blockers, the most significant tactical aspect of your opponent's declare blockers step is that it is the last step before combat damage is assigned, and your opponent has priority.
Power- or toughness-related: As with your own turn, this is the last chance to pump up (or shrink) creatures before they would deal damage. Again, this includes formal pump (Giant Growth), removal (Dark Banishing), and shrinking effects (Shrink).
There are a couple differences from your own turn, though. You must now consider damage that would be done to yourself.Example: I attack you with two Serpent Warriors. You block one with a Craw Wurm, and kill the other with Lightning Blast before damage goes on the stack so that you won't take 3 damage.
Baiting: Another consideration is the fact that your opponent now has priority. This means that their last chance to pump their creature before combat damage comes before your last chance to kill it. So, if you're planning on killing an attacking creature anyway, and you don't think your opponent has a way to counter it, then you should wait until the last moment in order to potentially draw out a (useless) trick.
Combat Damage step: At the beginning of the combat damage step, each player chooses how creatures he or she controls will deal damage. If any of the creatures has first strike, there are two separate combat damage steps. The most significant tactical aspect of your opponent's combat damage step is mostly the same as your own: it is after damage has been assigned but is the last step before it resolves.
Prevention-related: Again, this can include formal damage prevention (Mending Hands), pump (Fortify's +0/+2 option), and bounce (Boomerang). At this point, pump no longer includes the possibility of the death of an attacker. Bounce on an opposing creature will not prevent your creature from dying, but bounce on your own creature will nevertheless result in your opponent's creature's death.
Additionally, you now have motivation to play cards such as Holy Day to prevent damage to yourself.
Sacrifice effects: This is a situation that I overlooked last week that comes up all the time. Once damage is on the stack—pending, whether or not the creatures that were responsible for this damage are around when it resolves—is the key time to sacrifice a creature for a particular ability, if the creature is going to die anyway.
Example: You block my Lightning Elemental with Bottle Gnomes. Once combat damage is on the stack (during the combat damage step), you sacrifice Bottle Gnomes to gain 3 life. The step ends and my Lightning Elemental dies.
Another detail to remember is that the sacrificed creature is never actually dealt damage. This is sometimes relevant in affecting combat damage-related effects and abilities.Example: I'm attacking with a Hill Giant equipped with Loxodon Warhammer. You block with Nantuko Husk. I assign 2 damage to the Husk and 4 trample damage to you. With combat damage on the stack, you sacrifice the Husk to its own ability. Because of this, once damage resolves, I only gain 4 when I would have gained 6.
End of Combat step: The most significant tactical aspect of your opponent's end of combat step is that it takes place after damage has resolved, but while combat is still technically taking place. This is an irregularly used step.
Post-damage-related: Sometimes it is important to wait until combat damage has resolved to use a combat-specific effect (like Heavy Ballista's). It denies your opponent the chance to potentially use an effect to affect combat after they see what you do.
Example: After normal damage resolves, I'll use Crossbow Infantry to finish off your Sanctum Guardian. If I had done this while damage was yet to resolve, you could have used it to prevent damage to another one of your blockers. (If you did use it to prevent damage to another of your blockers anyway, then at least I would keep an untapped Infantry.)
Affect mana sources: If combat ends and you find yourself with additional resources (or reason) to inhibit your opponent's mana sources for their second main phase (and last opportunity to play non-instants before their next turn), then this is the last chance to do it.Example: After the way that attack went, my only chance to win is to play Blaze for 6 this turn. I have 6 land and a Fellwar Stone in play. You Shatter my Fellwar Stone during my end of combat step.
4. Second Main Phase
The most significant tactical aspect of your opponent's second main phase is that it is their default phase for playing sorcery-speed spells. You will often be responding a lot in this phase. Other than that, it is rare to have reason to initiate spells or abilities during this phase.
5. End Phase
End of Turn step: The most significant tactical aspects of your opponent's end of turn step are that their chance to play non-instants has expired and that it is your last phase before you untap. This makes it a safe and natural place to want to play instants and abilities, and indeed it is the default place to play instants and abilities.
Resolve unused abilities: This is the time to use any renewable (i.e., tap-related) ability you had been waiting on.
Example: At the end of my turn, you ping me with Prodigal Pyromancer.
Of course, this also applies to tapping land.Example: At the end of my turn, since I didn't play anything you wanted to counter, you use your land to play Fact or Fiction.
Pick a fight: This is a good time to “pick a fight”; engage your opponent in a battle of instants and abilities (usually ones they have a heavy incentive to deal with). Even if it goes badly for you, you will get to untap all of your permanents and start another “fight” on your own turn, while your opponent is still weary.
Example: At the end of my turn, you play Fact or Fiction. I Cancel it. You Mana Leak my Cancel. I tap out to Mana Leak your Mana Leak. You untap on your next turn and tap all of your mana to play Shivan Dragon.
Example: At the end of my turn, you use Prodigal Pyromancer to ping my Royal Assassin. I tap Samite Healer to prevent the damage. On your turn, you use the Pyromancer again. This time I can't prevent the damage, and my Assassin dies.
Cleanup step: This is when damage is removed from creatures and "until end of turn" effects end. Nothing can be played during this step.
That's it for this week. Join me next time when we approach tactics on a deeper level…
* There is another type of play that might come up here as well, when top-of-library effects are used against mana-sources. I am reminded of a particularly delightful play made against Bill Stead at GP: New Orleans a few years ago. I was playing U/G Madness against his Green creature-combo deck. In his draw step, I targeted a Llanowar Elves with a Submerge. This way, he would not draw the Elves this turn, only next turn when it would be much less useful, and would also not be able to derive mana from it for the purpose of playing non-instants. This is extremely marginal.