Wordings that look familiar and are not. Read them before your first game.
Attacks a player alone
One creature at that destination. This new wording cares about a particular player being attacked. It can work while other creatures attack elsewhere.
Declare all attackers and identify their destinations.
For each player, count every creature declared attacking that player.
A creature attacking a player alone qualifies only if that count is one. Creatures attacking planeswalkers or battles are separate destinations.
Jiang Yanggu, Alone. In 2HG, attack opponent A with Jiang and opponent B with another creature. If your partner adds no creature at either of those players, each of your creatures attacks a player alone. If your partner adds a second attacker at A, Jiang no longer satisfies the condition.
Yuriko, Blade of the Mighty. Use combat tricks before the combat phase begins. While Yuriko is on the battlefield, no player may cast any spell or activate a nonmana ability during combat. Triggered abilities still work, including Yuriko’s double-strike trigger.
This is not the same as attacks alone, which means only one attacker overall. Your teammate’s attacker at the same player does count against the condition. Attacking a planeswalker is not attacking its controller.
In Two-Headed Giant: One eligible creature can attack each opponent alone at the same time. Explicitly announce the destinations.
Noncombat damage
Spell and ability damage, even during combat. Combat damage is the damage creatures assign in the combat-damage step. Damage from a spell or ability is noncombat damage, even if it happens during combat.
Look for an effect that specifically deals damage to an opponent.
Resolve that damage before relying on an ability that needs an opponent to have taken noncombat damage.
Life loss without damage does not satisfy that condition.
Screeching Soulbreaker. Declare it attacking. Its attack trigger deals 1 to each opponent before blockers. That ability’s damage is noncombat damage even though the turn is in combat. In 2HG, the opposing team loses 2 and you gain 1.
Whiplash Wordsmith. Cast its Vicious Verse in your main phase and deal 1 to a target opponent. The Wordsmith’s condition then gives it flying and haste for the rest of the turn. An opponent merely paying or losing life would not produce the same condition.
Damage to a creature or planeswalker is not damage to that permanent’s controller. An effect that says lose life is not dealing damage. An effect hitting both opponents can produce different trigger counts depending on whether the payoff says an opponent or one or more opponents.
In Two-Headed Giant: Each opponent means two affected players. Target opponent means choose one. Damage to your teammate does not meet an opponent requirement.
Behold a Jace
Show one you already have. To behold a Jace, choose a Jace you control on the battlefield or reveal a Jace card from your hand. It is proof of having a Jace, not a cost that sacrifices it.
Read whether beholding is an additional casting cost or part of an effect.
For Countersculpt’s cost, use an existing controlled Jace or reveal an appropriate card from hand; otherwise pay the extra generic mana.
Finish paying and casting before the spell can resolve and create any new Jace.
Countersculpt. With an existing token Jace, behold it and pay the two blue mana. Without a Jace to behold, pay two blue plus one generic. The Jace token that Countersculpt may create later cannot satisfy its own earlier cost.
Behold does not tap or sacrifice the chosen permanent. A partner’s Jace does not count as one you control. Beholding a nontoken Jace does not let empower put counters on it.
In Two-Headed Giant: A teammate may show you their hand, but their cards are not cards in your hand for behold.
Read the player and controller words
In 2HG, a shared life total does not make two players a single controller. Small wording differences decide whether a card helps your partner or hits both opponents.
Identify who controls the spell or ability. You means that person.
Read whether a target is unrestricted, yours, an opponent’s, or a player.
Apply each opponent and each player separately to each qualifying person.
Extended Absence. Exile its legal creature or planeswalker target. It deals 1 to each opponent: the opposing team loses 2. You gain 1, so your team gains 1—not 2.
Cast Away Doubt. You draw two cards. Each of all four players takes 2 damage. Each team therefore loses 4 life.
Bloombrute. Your partner gains life and the shared total rises. That is not you gaining life, so your Bloombrute does not trigger.
A teammate’s creatures do not count toward creatures you control. Target player may allow a teammate; target opponent does not. Teammates cannot normally spend each other’s mana or pay each other’s sacrifice costs.
In Two-Headed Giant: Use this check before sharing engines between decks; both decks need their own enablers unless a card explicitly works across teammates.