In a game of Commander where sideboards don't exist, what happens when someone "learns" with divide by zero?
Is the player forced to "rummage"/cycle (discard then draw), or can they still opt to reveal then "fail to find" (a Lesson card from "outside the game")?
Also, is a player ever forced to reveal a card from their sideboard (when one exists) if they have no cards in hand to discard?
The mechanics article for Strixhaven: School of Mages clarifies that a player who "learns" can either reveal a Lesson card they own from outside the game and put it into their hand, or discard a card to draw a card, or do neither of these.
Note, however, that nothing in the rules for the Commander variant (under C.R. 903) changes what "outside the game" means. In general, under the comprehensive rules, a card "outside the game" means a card that's not in any of the game's zones, including a card in a sideboard (C.R. 400.11, 400.11a). But the players in an unsanctioned casual game can agree on modifications to the comprehensive rules (that is, "house rules") that address various game details, such as the meaning of "outside the game" or whether players can have sideboards. In sanctioned tournaments, a card "you own from outside the game" means only a card "you own" in a sideboard, even if the tournament uses the Commander variant (M.T.R. 3.16). Sideboards, though, are generally used only to "modify [a] deck between games of a match" (C.R. 100.4); the usual number of games in a match is given in C.R. 100.6a. See also this thread.
EDIT (after comment 4 was posted): Also, in the Commander variant, you can't bring a card you own into the game from outside it (whether by learning or otherwise) if—
the card has the same name as a card in your starting deck (including your commander [C.R. 903.3]) or a card you own in the current game, or
the card's color identity includes colors outside your commander's color identity
(C.R. 903.11).
EDIT (Jul. 23, 2021): Edited to conform to rule update with Adventures in the Forgotten Realms.
In a game of Commander where sideboards don't exist, what happens when someone "learns" with divide by zero?
Anything pertaining to cards "outside the game" needs either an official statement from the EDH rules committee or house rules in order to work in Commander. And the EDH rules committee did make an official statement about Learn : https://mtgcommander.net/index.php/2021/04/12/2021-april-update-strixhaven/.
In short, the "fetch a Lesson" part of Learn doesn't work in Commander unless your playgroup makes house rules to allow it. But the "rummage" option and the "do nothing" option work fine.
I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
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Is the player forced to "rummage"/cycle (discard then draw), or can they still opt to reveal then "fail to find" (a Lesson card from "outside the game")?
Also, is a player ever forced to reveal a card from their sideboard (when one exists) if they have no cards in hand to discard?
Thanks!
I used to be a demigod, but now I'm an omnimage
Note, however, that nothing in the rules for the Commander variant (under C.R. 903) changes what "outside the game" means. In general, under the comprehensive rules, a card "outside the game" means a card that's not in any of the game's zones, including a card in a sideboard (C.R. 400.11, 400.11a). But the players in an unsanctioned casual game can agree on modifications to the comprehensive rules (that is, "house rules") that address various game details, such as the meaning of "outside the game" or whether players can have sideboards. In sanctioned tournaments, a card "you own from outside the game" means only a card "you own" in a sideboard, even if the tournament uses the Commander variant (M.T.R. 3.16). Sideboards, though, are generally used only to "modify [a] deck between games of a match" (C.R. 100.4); the usual number of games in a match is given in C.R. 100.6a. See also this thread.
EDIT (after comment 4 was posted): Also, in the Commander variant, you can't bring a card you own into the game from outside it (whether by learning or otherwise) if—
EDIT (Jul. 23, 2021): Edited to conform to rule update with Adventures in the Forgotten Realms.
In short, the "fetch a Lesson" part of Learn doesn't work in Commander unless your playgroup makes house rules to allow it. But the "rummage" option and the "do nothing" option work fine.