A replacement effect does not respond to an event; it changes the event before it happens. The signal word is 'instead': if an effec...
A replacement effect does not respond to an event; it changes the event before it happens. The signal word is 'instead': if an effect says something happens instead of something else, the original event never occurs — it is modified on its way in. That is fundamentally different from a triggered ability, which waits for the event and then reacts to it on the stack.
The example card, read exactly
Hardened Scales — Enchantment (mana cost {G}):
If one or more +1/+1 counters would be put on a creature you control, that many plus one +1/+1 counters are put on it instead.
Rules answers live and die on exact wording, which is why every card named on this page can be checked in the MTGBrew card database before you rely on it at a table.
How it plays out, step by step
Watch the difference on one play. A creature enters with a +1/+1 counter while a replacement effect that adds counters is on the battlefield: the creature simply arrives with more counters — there is no trigger to respond to, no moment where it existed with fewer. If two replacement effects could both apply to the same event, the affected player — or the controller of the affected object — chooses the order they apply in, and each effect can only apply to a given event once. The practical lesson: you cannot 'respond to' a replacement effect, because by the time anything could respond, the modified event has already happened in its new form.
When you build around interactions like this, the MTGBrew AI deck builder keeps the list honest — and the MTGBrew Learn guides collect more of these plain-English rules walk-throughs in one place.
If this topic is on your bench, keep going with Dragons Deck Tech: How the Archetype Works and Wins in Commander — it covers the neighbouring decisions from a different angle, and the two pieces together give you a fuller picture than either one alone.
The short version
Watch the difference on one play. Everything else in this article is the working-out — and at a real table, the working-out is what settles the argument, so walk the steps in order and let the text of the cards involved make the final call.
Keep reading
Keep reading: Dragons Deck Tech: How the Archetype Works and Wins in Commander and Precon Upgrade Guide: Ellivere of the Wild Court — What to Cut, What to Add, by Role.
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