Mystic Forge

• WWizards of the Coast2019-07-12
  Official Ruling
  Because you never "cast" a land card, Mystic Forge doesn't allow you to play an artifact land from the top of your library.
• WWizards of the Coast2019-07-12
  Official Ruling
  Mystic Forge lets you look at the top card of your library whenever you want (with one restriction—see below), even if you don't have priority. This action doesn't use the stack. Knowing what that card is becomes part of the information you have access to, just like you can look at the cards in your hand.
• WWizards of the Coast2019-07-12
  Official Ruling
  You still pay all costs for a spell you cast from your library, including additional costs. You may also pay alternative costs.
• WWizards of the Coast2019-07-12
  Official Ruling
  You must follow the normal timing permissions and restrictions of the spells you cast from your library.
• WWizards of the Coast2019-07-12
  Official Ruling
  If the top card of your library changes while you're casting a spell, playing a land, or activating an ability, you can't look at the new top card until you finish doing so. This means that if you cast the top card of your library, you can't look at the next one until you're done paying for that spell.
• WWizards of the Coast2019-07-12
  Official Ruling
  If the top card of your library has a morph ability, you can cast it face down from the top of your library, even if it's normally not a colorless card.

Rulings via Scryfall

• WO
  Wizards of the Coast2019-08-26

  Mystic Forge was restricted in vintage on August 26, 2019. Recently, Karn, the Great Creator and Mystic Forge have turbo-charged Shops decks by giving them more early-game lockout potential, a tutorable win condition, and card advantage in prolonged games. Therefore, in order to make Shops decks more interactive in the early game and more attackable in a prolonged game, we are restricting Mystic Forge. Source: https://magic.wizards.com/en/news/news/august-26-2019-banned-and-restricted-announcement-2019-08-26