If Retriever Phoenix somehow dies with its triggered ability on the stack, you may return it to the battlefield with that triggered ability.

Retriever Phoenix must be in your graveyard at the moment you're instructed to learn if you want to use the last ability to return it to the battlefield.

If instructed to learn, you may do nothing. Discarding a card and putting a Lesson card into your hand are both optional.

If a card is brought into the game from outside the game, it will stay in the game until it ends or until its owner leaves the game, whichever comes first.

If you bring a card into the game from outside the game in a Constructed tournament (or a Constructed game played under tournament conditions), you may select only a card from your sideboard. In casual Constructed games, the card comes from your collection.

If you bring a card into the game from outside the game in a Sealed Deck or Booster Draft event, you may select only a card that is in the card pool that you built your deck from.