We’re looking at that second ability today: “Exile the top card of your library. You may play it this turn.” The ability pretty much spells out what you do. You exile the top card of your library, and until the end of your turn, you can play it. If it’s land and you haven’t used up all of your land drops, you can play it from exile. If it’s a nonland card, you can cast it like normal. Unlike other effects (like Unexpected Results), you do have to pay the mana cost of the spell. So you can’t use Chandra to try to cheat out some expensive spell and hope to be able to cast it even if you’re tapped out.
If you’re unable or you just don’t want to play the exiled card this turn, it just stays in exile like any other exiled card, and you won’t be able to cast it later.
But hey, it’s not all bad. If you exile a spell with X in the cost with Chandra’s ability, like Volcanic Geyser, you’ll be able to have it deal damage. If you cast a spell without paying its mana cost, X has to be 0. But with Chandra, since you’re just casting the spell (just from exile instead of your hand), you can pay something for X and have the Geyser deal some damage.
Today’s Rules Tip written by Nathan Long