If a spell or ability with one or more targets states that you "may pay" some amount of , and each permanent that it targets has become an illegal target, the spell or ability won't resolve. You can't pay any even if you want to.

Some spells and abilities that give you may require targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won't resolve. You won't get any .

If the copied creature is copying something else, then Aurora Shifter becomes a copy of whatever that creature copied (with the listed exception).

Energy counters are a kind of counter that a player may have. They're not associated with any specific permanents.

If the copied creature has  in its mana cost, X is 0.

Keep track of how many energy counters each player has. Potential ways to track this include writing theme down on paper or using dice, but any method that is clear and mutually agreeable is fine. (At higher levels of tournament play, dice may not be allowed for tracking counters that players have.)

Some triggered abilities that state that you "may pay" a certain amount of describe an effect that happens "If you do." In that case, no player may take actions to try to stop the ability's effect after you make your choice. If the payment is followed by the phrase "When you do," then you'll choose any targets for that reflexive triggered ability and put it on the stack before players can take actions.

Energy counters aren't mana. They don't go away as steps, phases, and turns end, and effects that add mana "of any type" can't give you energy counters.

Because Aurora Shifter isn't entering the battlefield when it becomes a copy of another creature, any "When [this creature] enters the battlefield" or "[This creature] enters the battlefield with" abilities of the copied creature won't apply.

is the energy symbol. It represents one energy counter.

You don't choose a target for Aurora Shifter's last ability at the time it triggers. Rather, a second "reflexive" ability triggers when you pay this way. You choose a target for that ability as it goes on the stack. Each player may respond to this triggered ability as normal.

Aurora Shifter copies exactly what was printed on the original creature and nothing else, with the listed exception (unless that permanent is copying something else; see below). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, and so on.

If an effect says you get one or more , you get that many energy counters. To pay one or more , you lose that many energy counters. You can't pay more energy counters than you have. Any effects that interact with counters a player gets, has, or loses can interact with energy counters.

Some triggered abilities state that you "may pay" a certain amount of . You can't pay that amount multiple times to multiply the effect. You simply choose whether or not to pay that amount of as the ability resolves.