Abilities

Can triggered abilities trigger on an opponent's turn without Reaction?

Yes. A triggered ability is put on the chain whenever its condition is met, regardless of whose turn it is, so it does not need Reaction.^{[383.3][383.3.c]}

It first enters as a pending item: it has triggered but must still complete the required choices and costs before it finalizes and can resolve.^[400][400.1] If its controller declines an initial "you may," that pending ability is removed from the chain and considered not to have triggered.^[383.3.a.2]

Can I react to triggered and activated abilities?

Yes, except for triggered and activated abilities that use Add.

Triggered and activated abilities use the chain and become finalized chain items.^{[377.3][383.3][406.2]} Once no pending items remain, you have an opportunity to play Reactions before those abilities resolve.^{[337.4][406.4]}

An ability that adds resources instead resolves immediately after finalization without passing priority.^{[429.2][429.2.a]}

Does removing a triggered ability's source stop that ability?

No. A triggered ability finalizes before opponents can react to it.^{[406.2][406.4]} Removing its source is not a counter instruction, so the ability remains on the chain and executes like a spell.^{[425.1][425.2.a][406.5]}

An instruction can still do nothing if it needs current information from the missing source. Source referents such as "here," "my," or "this battlefield" are normally checked when that instruction executes.^{[359.3.f.1][359.3.f.2]} Information referenced from a triggered ability's trigger condition is instead recorded when the condition is fulfilled and remains available if the source later moves or leaves.^[359.3.f.3] If an instruction instead needs the source's current location and the source is no longer on the board, that information is unavailable, so the instruction cannot execute.^[359.3.e.12]

For example, if an opponent responds to Ashe, Focused's play effect with Star-Crossed and returns Ashe to her owner's hand, her ability still resolves because none of its instructions needs Ashe to remain on the board. By contrast, if Star-Crossed returns Vi, Peacekeeper before her attack trigger resolves, Vi cannot Stun an enemy unit "here" because she no longer has a location for "here" to reference.

When does the game check an "if" condition in a triggered ability?

An "if" condition immediately after the trigger condition is checked after the triggering event, while one after an effect instruction is checked during resolution.

• Immediately after the trigger condition: A trigger condition begins with "When," "At," or "The [Nth] time," and an additional conditional statement immediately following it is part of that condition.^{[383.2.a][383.2.a.1]} The game checks those requirements after the triggering event has been processed.^[383.2.c] If they are not satisfied then, the ability is not placed on the chain.^[383.3] For example, Gutter Palace checks the hand and battlefield quantities in its "if" condition at the start of the Beginning Phase; if either is wrong then, its ability is not placed on the chain.
• After an effect instruction: The "if" condition is part of the effect, so the ability is placed on the chain and checks that condition when it resolves.^{[383.2.b][359.3.d][406.5]} For example, Loose Cannon first instructs its controller to draw 1 and then checks hand size, so its ability is placed on the chain at the start of every Beginning Phase and checks the "if" condition during resolution.

When do I choose whether to use a triggered ability that says "you may"?

If "you may" begins the effect, choose during finalization; if it appears later, choose during resolution.

• Initial "you may": Choose while the pending ability is being finalized.^[383.3.a] If you decline, the ability is removed from the chain and considered not to have triggered.^[383.3.a.2] If you proceed, make its required choices and pay any costs incurred during finalization before the ability becomes a finalized chain item.^{[402.2][404.1][406.2]} For example, Tideturner begins its effect with "you may," so its controller decides whether to use the ability during finalization.
• Later "you may": If another effect instruction comes first, make the optional choice when resolution reaches that later instruction.^[383.3.a.3] For example, Ornn, Blacksmith first instructs its controller to look at cards, then says "You may reveal a gear," so that choice is made during resolution.

When do I pay costs in a triggered ability?

The "[do X] to [do Y]" form creates a cost within instructions: [do X] is the cost and [do Y] is its effect.^[740.4.a] Pay [do X] during finalization if it begins the triggered effect or immediately follows an initial "you may."^{[383.3.b][383.3.b.1][740.4.a.2]} Pay it during resolution if it appears later.^{[740.4.a.2.a]}

If that cost is due during finalization and you decline to pay it, the pending ability is removed from the chain without finalizing.^[404.2]

The "[do X]. If you do, [do Y]" form does not create a cost within instructions. The [do X] instruction is executed during resolution.^[406.5]

For example, Vayne, Hunter says "When I conquer, you may pay 1 to return me to my owner's hand." The "pay 1 to return me" form creates a cost within instructions immediately after the initial "you may," so you pay it during finalization. If you pay, the ability finalizes and opponents may react before it resolves.^[406.4]

By contrast, Insightful Investigator first has an opponent reveal their hand and only then offers a payment, so that cost within instructions appears later and you pay it during resolution.