A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.

If a permanent has in its mana cost, X is considered to be 0.

Karn's last ability can't get a face-down card in exile, even if you know it's an artifact card.

In a casual game, a card you choose from outside the game comes from your personal collection. In a tournament event, a card you choose from outside the game must come from your sideboard. You may look at your sideboard at any time.

Karn's first ability affects only artifacts on the battlefield. Activated abilities that work in other zones can still be activated.

Activated abilities contain a colon. They're generally written "[Cost]: [Effect]." Some keyword abilities are activated abilities and will have colons in their reminder text, such as equip. Triggered abilities (starting with "when," "whenever," or "at") are unaffected by Karn.