Rule 118.12

Some spells, activated abilities, and triggered abilities read, “[Do something]. If [a player] [does, doesn’t, or can’t], [effect].” Or “[A player] may [do something]. If [that player] [does, doesn’t, or can’t], [effect].” The action [do something] is a cost, paid when the spell or ability resolves. The “If [a player] [does, doesn’t, or can’t]” clause checks whether the player chose to pay an optional cost or started to pay a mandatory cost, regardless of what events actually occurred. Example: You control Standstill, an enchantment that says “When a player casts a spell, sacrifice this enchantment. If you do, each of that player’s opponents draws three cards.” A spell is cast, causing Standstill’s ability to trigger. Then an ability is activated that exiles Standstill. When Standstill’s ability resolves, you’re unable to pay the “sacrifice Standstill” cost. No player will draw cards. Example: Your opponent has cast Gather Specimens, a spell that says “If a creature would enter the battlefield under an opponent’s control this turn, it enters under your control instead.” You control a face-down Dermoplasm, a creature with morph that says “When this creature is turned face up, you may put a creature card with morph from your hand onto the battlefield face up. If you do, return this creature to its owner’s hand.” You turn Dermoplasm face up, and you choose to put a creature card with morph from your hand onto the battlefield. Due to Gather Specimens, it enters the battlefield under your opponent’s control instead of yours. However, since you chose to pay the cost, Dermoplasm is still returned to its owner’s hand.