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Primal Amulet's last ability triggers once you've completed casting a spell. Notably, you can't use Primal Wellspring to pay for the spell that gives Primal Amulet its fourth counter.
Primal Wellspring's delayed triggered ability can copy the spell even if that spell is countered before the ability resolves.
The copy will have the same targets as the spell it's copying unless you choose new ones. You may change any number of the targets, including all of them or none of them. If, for one of the targets, you can't choose a new legal target, then it remains unchanged (even if the current target is illegal).
If a fourth charge counter is put on Primal Amulet by something other than the resolution of its ability (as modified by any applicable replacement effects), you won't be able to remove those counters and transform it yet. You'll have to wait until you cast an instant or sorcery spell again.
If a copy is created, you control the copy. That copy is created on the stack, so it's not "cast." Abilities that trigger when a player casts a spell won't trigger. The copy will then resolve like a normal spell, after players get a chance to cast spells and activate abilities.
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.
You can't choose to pay any additional costs for the copy. However, effects based on any additional costs that were paid for the original spell are copied as though those same costs were paid for the copy too. For example, if you sacrifice a 3/3 creature to cast Fling, and you copy it, the copy of Fling will also deal 3 damage to its target.
Effects that reduce the generic mana cost of a spell can't reduce that spell's colored mana requirements.
If the copied spell has an X whose value was determined as it was cast, the copy has the same value of X.
If more than one mana produced by a Primal Wellspring is spent to cast a single instant or sorcery spell, the delayed triggered ability associated with each mana spent will trigger. That many copies will be created. It doesn't matter if this mana was produced by one Primal Wellspring or by multiple Primal Wellsprings.
The delayed triggered ability from Primal Wellspring's mana ability will trigger even if Primal Wellspring leaves the battlefield before that mana is spent.
If the copied spell is modal (that is, it says "Choose one —" or the like), the copy will have the same mode. You can't choose a different one.
Any instant or sorcery spell you spend the mana on will be copied, not just one that requires targets.
The mana produced by Primal Wellspring can be spent on anything, not just an instant or sorcery spell.
