Divide by Zero — ruling 1

If an effect returns a copy of a spell to its owner’s hand, the copy ceases to exist.

Divide by Zero — ruling 2

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.

Divide by Zero — ruling 3

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.

Divide by Zero — ruling 4

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.

Divide by Zero — ruling 5

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

Divide by Zero : règles, fonctionnement et interactions (MTG)

Divide by Zero is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Return target spell or permanent with mana value 1 or greater to its owner's hand. Learn. (You may reveal a Lesson card you own from outside the game and put it into your hand, or discard a card to draw a card.)

How it works : Divide by Zero

Original English text — Wizards of the Coast / Scryfall

#1April 16, 2021· Wizards of the Coast
If an effect returns a copy of a spell to its owner’s hand, the copy ceases to exist.
No other card is named in this ruling.
#2April 16, 2021· Wizards of the Coast
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.
No other card is named in this ruling.
#3April 16, 2021· Wizards of the Coast
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.
No other card is named in this ruling.
#4April 16, 2021· Wizards of the Coast
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.
No other card is named in this ruling.
#5April 16, 2021· Wizards of the Coast
If instructed to learn, you may do nothing. Discarding a card and putting a Lesson card into your hand are both optional.
No other card is named in this ruling.