Jorn, God of Winter // Kaldring, the Rimestaff

• WWizards of the Coast2021-02-05
  Official Ruling
  You'll still pay all costs to cast a spell this way, including any additional costs. You may also pay alternative costs if any are available.
• WWizards of the Coast2021-02-05
  Official Ruling
  Even though you may play the card, it isn't in your hand, so you can't, for example, discard it or foretell it.
• WWizards of the Coast2021-02-05
  Official Ruling
  If an effect allows you to play a specific modal double-faced card, you may cast it as a spell or play it as a land, as determined by which face you choose to play. If an effect allows you to cast (rather than "play") a specific modal double-faced card, you can't play it as a land.
• WWizards of the Coast2021-02-05
  Official Ruling
  Jorn's triggered ability will untap Jorn itself.
• WWizards of the Coast2021-02-05
  Official Ruling
  The symbol is a generic mana symbol. It represents a cost that can be paid by one mana that was produced by a snow source. That mana can be any color or colorless.
• WWizards of the Coast2021-02-05
  Official Ruling
  In the Commander variant, a double-faced card's color identity is determined by the mana costs and mana symbols in the rules text of both faces combined. If either face has a color indicator or basic land type, those are also considered.
• WWizards of the Coast2021-02-05
  Official Ruling
  Snow isn't a type of mana. If an effect says you may spend mana as though it were any type, you can't pay for using mana that wasn't produced by a snow source.
• WWizards of the Coast2021-02-05
  Official Ruling
  Snow is a supertype, not a card type. It has no rules meaning or function by itself, but spells and abilities may refer to it.
• WWizards of the Coast2021-02-05
  Official Ruling
  If an effect instructs a player to choose a card name, the name of either face may be chosen. If that effect or a linked ability refers to a spell with the chosen name being cast and/or a land with the chosen name being played, it considers only the chosen name, not the other face's name.
• WWizards of the Coast2021-02-05
  Official Ruling
  You don't play the card as Kaldring's activated ability is resolving; rather, that ability gives you permission to play that card that turn. You must follow the normal timing permissions and restrictions for the card when you play it. To play a land this way, you must have a land play available. Notably, if you activate Kaldring's ability during an opponent's turn, you won't be able to cast a spell that way unless it has flash and you won't be able to play a land that way at all.
• WWizards of the Coast2021-02-05
  Official Ruling
  Some cards have additional effects for each spent to cast them. You can cast these spells even if you don't spend any snow mana to cast them; their additional effects simply won't do anything.
• WWizards of the Coast2021-02-05
  Official Ruling
  The mana value of a modal double-faced card is based on the characteristics of the face that's being considered. On the stack and battlefield, consider whichever face is up. In all other zones, consider only the front face. This is different than how the mana value of a transforming double-faced card is determined.
• WWizards of the Coast2021-02-05
  Official Ruling
  The Kaldheim set doesn't have any cards with mana costs that include , but some previous sets do. If an effect says such a spell costs less to cast, that reduction doesn't apply to any costs. This is also true for activated abilities that include in their activation costs and effects that reduce those costs.
• WWizards of the Coast2021-02-05
  Official Ruling
  If you don't end up playing the card, it remains in your graveyard. You can't play it on future turns unless you target it again with Kaldring's ability (or something else lets you play it).
• WWizards of the Coast2021-02-05
  Official Ruling
  A snow permanent card is any artifact, creature, enchantment, land, or planeswalker card with the supertype snow.
• WWizards of the Coast2021-02-05
  Official Ruling
  If an effect allows you to play a land or cast a spell from among a group of cards, you may play or cast a modal double-faced card with any face that fits the criteria of that effect.
• WWizards of the Coast2021-02-05
  Official Ruling
  If an effect puts a double-faced card onto the battlefield, it enters with its front face up. If that front face can't be put onto the battlefield, it doesn't enter the battlefield.
• WWizards of the Coast2021-02-05
  Official Ruling
  There is a single triangle icon in the top left corner of the front face. There is a double triangle icon in the top left corner of the back face.
• WWizards of the Coast2021-02-05
  Official Ruling
  A modal double-faced card can't be transformed or be put onto the battlefield transformed. Ignore any instruction to transform a modal double-faced card or to put one onto the battlefield transformed.
• WWizards of the Coast2021-02-05
  Official Ruling
  To determine whether it is legal to play a modal double-faced card, consider only the characteristics of the face you're playing and ignore the other face's characteristics.

Rulings via Scryfall