Ivy, Gleeful Spellthief

• WWizards of the Coast2022-09-09
  Official Ruling
  If the spell that's copied 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.
• WWizards of the Coast2022-09-09
  Official Ruling
  The second ability triggers whenever a player casts a spell that targets only one creature and no other object or player. It doesn't matter who controls the creature.
• WWizards of the Coast2022-09-09
  Official Ruling
  The copy is created even if the spell that caused the ability to trigger has been countered by the time the ability resolves. The copy resolves before the original spell.
• WWizards of the Coast2022-09-09
  Official Ruling
  The controller of a copy 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. Notably, if the spell you copy was kicked, the copies will also be kicked.
• WWizards of the Coast2022-09-09
  Official Ruling
  The copy that Ivy, Gleeful Spellthief's second ability creates is created on the stack, so it's not "cast." Other abilities that trigger when a player casts a spell won't trigger.
• WWizards of the Coast2022-09-09
  Official Ruling
  If the spell that's copied has an X whose value was determined as it was cast, the copy has the same value of X.
• WWizards of the Coast2022-09-09
  Official Ruling
  If a spell has multiple targets but all of them are the same creature, Ivy, Gleeful Spellthief's second ability will trigger.
• WWizards of the Coast2022-09-09
  Official Ruling
  If Ivy, Gleeful Spellthief isn't a legal target for the spell, no copy can be created.

Rulings via Scryfall