421.4. If the loop contains only mandatory actions, the game ends in a draw. (See rule 102.4b.)

421.5. If the loop contains optional actions controlled by different players and these actions don’t depend on one another, the active player chooses a number. In APNAP order, the nonactive players can each either agree to that number or choose a higher number. Note that this rule applies even if the actions could exist in separate loops rather than in a single loop.

422.1. If a player realizes that he or she can’t legally take an action after starting to do so, the entire action is reversed and any payments already made are canceled. No abilities trigger and no effects apply as a result of an undone action. If the action was playing a spell, the spell returns to the zone it came from. The player may also reverse any legal mana abilities played while making the illegal play, unless mana from them or from any triggered mana abilities they triggered was spent on another mana ability that wasn’t reversed. Players may not reverse actions that moved cards to a library or from a library to any zone other than the stack.

422.2. When reversing illegal spells and abilities, the player who had priority retains it and may take another action or pass. The player may redo the reversed action in a legal way or take any other action allowed by the rules.

423.1. A player draws a card by putting the top card of his or her library into his or her hand. This is done as a game action during each player’s draw step. It may also be done as part of a cost or effect of a spell or ability.

423.2. Cards may only be drawn one at a time. If a player is instructed to draw multiple cards, that player performs that many individual card draws.

423.2a If an effect instructs more than one player to draw cards, the active player performs all of his or her draws first, then each other player in turn order does the same.

432.2b If an effect instructs more than one player to draw cards in a Two-Headed Giant game, first the primary player (seated on the right) on the active team performs all of his or her draws, then the secondary player on that team performs all of his or her draws, then the nonactive team does the same.

423.3. If there are no cards in a player’s library and an effect offers that player the choice to draw a card, that player may choose to do so. See rule 413.2c.

423.4. A player who attempts to draw a card from an empty library loses the game the next time a player would receive priority. (This is a state-based effect. See rule 420.5g.)

423.5. If an effect moves cards from a player’s library to that player’s hand without using the word “draw,” the player has not drawn those cards. This makes a difference for abilities that trigger on drawing cards or that replace card draws, as well as if the player’s library is empty.

423.6. Some effects replace card draws.

423.6a An effect that replaces a card draw is applied even if no cards could be drawn because there are no cards in the affected player’s library.

423.6b If an effect replaces a draw within a sequence of card draws, the replacement effect is completed before resuming the sequence.

423.6c Some effects perform additional actions on a card after it’s drawn. If the draw is replaced, the additional action is not performed on any cards that are drawn as a result of that replacement effect or any subsequent replacement effects.

500.1. Some effects restrict declaring attackers or blockers in combat or require certain creatures to be declared as attackers or blockers. (See rule 308, “Declare Attackers Step,” and rule 309, “Declare Blockers Step.”) A restriction is an effect that says a creature can’t block (or attack) or it can’t block (or attack) unless some condition is met. A requirement is an effect that says a creature must block (or attack) or it must block (or attack) if some condition is met.

500.2. As part of declaring attackers, the active player checks each creature he or she controls to see whether it must attack, can’t attack, or has some other attacking restriction or requirement. If such a restriction or requirement conflicts with the proposed attack, the attack is illegal, and the active player must then propose another set of attacking creatures. (Tapped creatures and creatures with unpaid costs to attack are exempt from effects that would require them to attack.)

Example: A player controls two creatures, each with a restriction that states “[This creature] can’t attack alone.” It’s legal to declare both as attackers.

Example: A player controls two creatures: one that “attacks if able” and one with no abilities. An effect states, “No more than one creature may attack each turn.” The only legal attack is for just the creature that “attacks if able” to attack. It’s illegal to attack with the other creature, attack with both, or attack with neither.

500.3. As part of declaring blockers, the defending player checks each creature he or she controls to see whether it must block, can’t block, or has some other blocking restriction or requirement. If such a restriction or requirement conflicts with the proposed set of blocking creatures, the block is illegal, and the defending player must then propose another set of blocking creatures. (Tapped creatures and creatures with unpaid costs to block are exempt from effects that would require them to block.)

500.4. A restriction conflicts with a proposed set of attackers or blockers if it isn’t being followed. A requirement conflicts with a proposed set of attackers or blockers if it isn’t being followed and (1) the requirement could be obeyed without violating a restriction and (2) doing so will allow the total number of requirements that the set obeys to increase.

500.5. When determining what requirements could be obeyed without violating restrictions, you don’t need to consider any options for a creature that don’t satisfy a requirement on it. But you do need to consider any options for any creature(s) that will satisfy a requirement, as long as the total number of obeyed requirements is increased (even if the option means not obeying another requirement that was previously met).

Example: A player controls one creature that “blocks if able” and another creature with no abilities. An effect states, “Creatures can’t be blocked except by two or more creatures.” The creature with no abilities isn’t required to block. It’s legal to declare both creatures as blockers, or to declare neither creature as a blocker, but illegal to block with only one of the two.

501.1. Evasion abilities restrict what can block an attacking creature. These are static abilities that modify the declare blockers step of the combat phase.