Once and Future — ruling 1

You choose which card will be returned to your hand and which will be put on top of your library as you choose those cards as targets, not as Once and Future resolves.

Once and Future — ruling 2

If one of the two target cards is an illegal target by the time Once and Future resolves, you'll put the other in the appropriate zone and exile Once and Future. If both targets are illegal, the spell won't resolve and you won't exile Once and Future.

Once and Future — ruling 3

If an instant or sorcery spell has an adamant ability, you perform the spell's instructions in order. You don't perform the adamant instruction before the spell resolves or before any other effects printed above it.

Once and Future — ruling 4

If an effect allows you to cast a spell without paying its mana cost, you can't choose to cast it and pay unless another rule or effect allows you to cast that spell for a cost. Similarly, you can't waive a cost reduction unless that effect says you may.

Once and Future — ruling 5

Adamant effects check what mana was actually spent to cast a spell. If an effect allows you to spend mana “as though it were mana” of any color or type, that allows you to spend mana you couldn't otherwise spend, but it doesn't change what mana you spent to cast the spell.

Once and Future — ruling 6

If you copy a spell that has an adamant ability, no mana was spent to cast the copy at all, so that ability won't apply.

Once and Future : règles, fonctionnement et interactions (MTG)

Once and Future 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 card from your graveyard to your hand. Put up to one other target card from your graveyard on top of your library. Exile Once and Future. Adamant — If at least three green mana was spent to cast this spell, instead return those cards to your hand and exile Once and Future.

How it works : Once and Future

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2019· Wizards of the Coast
You choose which card will be returned to your hand and which will be put on top of your library as you choose those cards as targets, not as Once and Future resolves.
No other card is named in this ruling.
#2October 4, 2019· Wizards of the Coast
If one of the two target cards is an illegal target by the time Once and Future resolves, you'll put the other in the appropriate zone and exile Once and Future. If both targets are illegal, the spell won't resolve and you won't exile Once and Future.
No other card is named in this ruling.
#3October 4, 2019· Wizards of the Coast
If an instant or sorcery spell has an adamant ability, you perform the spell's instructions in order. You don't perform the adamant instruction before the spell resolves or before any other effects printed above it.
No other card is named in this ruling.
#4October 4, 2019· Wizards of the Coast
If an effect allows you to cast a spell without paying its mana cost, you can't choose to cast it and pay unless another rule or effect allows you to cast that spell for a cost. Similarly, you can't waive a cost reduction unless that effect says you may.
No other card is named in this ruling.
#5October 4, 2019· Wizards of the Coast
Adamant effects check what mana was actually spent to cast a spell. If an effect allows you to spend mana “as though it were mana” of any color or type, that allows you to spend mana you couldn't otherwise spend, but it doesn't change what mana you spent to cast the spell.
No other card is named in this ruling.
#6October 4, 2019· Wizards of the Coast
If you copy a spell that has an adamant ability, no mana was spent to cast the copy at all, so that ability won't apply.
No other card is named in this ruling.