In every zone other than the stack, Twice Upon a Time uses only its non-Adventure characteristics, and you ignore its alternative characteristics in those cases. For example, while it's in your graveyard, its mana value is 6.

If an effect copies an Adventure spell, that copy is exiled as it resolves. It ceases to exist as a state-based action; it's not possible to cast the copy from exile.

You must still follow any relevant timing rules for the non-Adventure spell you cast from exile. Normally, you'll be able to cast it only during your main phase while the stack is empty.

When casting a spell as an Adventure, use the alternative characteristics and ignore all of the card's normal characteristics. The spell's color, mana cost, mana value, and so on are determined by only those alternative characteristics. If the spell leaves the stack, it immediately resumes using its normal characteristics.

If an adventurer card ends up in exile for any other reason than by exiling itself while resolving, it won't give you permission to cast it as a non-Adventure spell.

Casting a card as an Adventure isn't casting it for an alternative cost. Effects that allow you to cast a spell for an alternative cost or without paying its mana cost may allow you to apply those to the Adventure.

If an effect instructs you to choose a card name, you may choose the alternative Adventure name. Consider only the alternative characteristics to determine whether that is an appropriate name to choose.

An effect may refer to a card, spell, or permanent that "has an Adventure." This refers to a card, spell, or permanent that has an adventurer card's set of alternative characteristics, even if they're not being used and even if that card was never cast as an Adventure.

If you cast an adventurer card as an Adventure, use only its alternative characteristics to determine whether it's legal to cast that spell.

If a spell is cast as an Adventure, its controller exiles it instead of putting it into its owner's graveyard as it resolves. For as long as it remains exiled, that player may cast it as a non-Adventure spell. If an Adventure spell leaves the stack in any way other than resolving (most likely by being countered or by failing to resolve because its targets have all become illegal), that card won't be exiled and the spell's controller won't be able to cast it as a permanent later.