Fourth Edition/Rules changes

The rules of Magic: The Gathering had undergone a series of changes since they were first released, due in part to their original incompleteness. The complexity of card interactions forced rulings to be made by interpreting the cards; several of these rulings were now general enough to affect many cards or situations, and the most important of these rulings appeared in the Pocket Players' Guide of the Fourth Edition.

Land

 * Playing a land is not a fast effect, so it cannot be done in response to anything, nor can it be responded to.
 * If a land has a special ability that does not require an activation cost at all, that ability is always "on", even when the land is tapped.

Damage prevention

 * A single damage prevention step occurs after resolving any single spell or effect that destroys or buries one or more permanents (except in the case of "at end of turn", which are discussed under "Rules about phases" and triggered effects which are discussed under "Triggered Effects"); all permanents buried or destroyed by that spell or effect are processed in that single damage prevention step.
 * A single damage prevention step also occurs after resolving a batch of spells and/or effects in which any creature or player is damaged or in which any creature's toughness is lowered to the point where it takes lethal damage.
 * Finally, a single damage prevention step occurs after resolving a damage dealing step in combat (either damage dealing from first strike or normal damage dealing) in which any creature or player is damaged.
 * A creature has taken "lethal damage" whenever it has taken damage equal or greater than its current toughness (including having its current toughness reduced to zero or less)

The Graveyard and Going to the Graveyard

 * A permanent is "on the way to the graveyard" only during a damage prevention step and only if it has taken lethal damage or it is being sent to the graveyard due to a destroy or bury spell or effect.
 * Cards sent to the graveyard are always put on the top of the graveyard. If multiple cards go to the graveyard at the same time, the owner of the cards chooses the order in which they are put in the graveyard.
 * Token creatures sent to the graveyard are momentarily put in the graveyard before being removed from the game; hence, token creatures sent to the graveyard generate effects that are triggered by creatures being put into the graveyard.
 * A player cannot reorder cards in the graveyard unless a spell or effect specifically allows or instructs to do so.

Cards that Are Out of Play

 * Unless specifically stated, spells and effects do not interact with cards that are not in play. For example, a spell that says "destroy all islands", only affects Islands in play.
 * Cards that are not in play have no memory of whether they were ever in play nor of anything that may have happened to them while they were in play (including the effects of interrupt spells).

Rules about Phases

 * The “active player" is the player whose turn it is.
 * Except during the untap and Heal Creatures phases, when the active player says "I’m done with the current phase”, the non-active player can declare fast effects in response to this announcement. Responding to such an announcement cancels the ending of the current phase and thus gives the active player additional opportunities to take actions during that phase.
 * Certain effects or required actions can only be used or must be used during a particular phase (for example, paying upkeep costs during upkeep or drawing a card during the draw phase).
 * Any such effect or required action that does not have an activation cost can only be used once during that phase.
 * Play all such effects and required actions as instants. Such effects and required actions can be played in any order provided that all such required actions are played or the source of such actions is removed before the phase ends.
 * You must always pay the entire upkeep cost on a permanent or none of it.
 * If a permanent has an upkeep cost, you may not use any ability of that permanent that requires an activation cost until you have paid the upkeep cost of that permanent.
 * Untap costs are an exception to the once-per-phase rule; untap costs can be paid multiple times during upkeep. You must always pay the entire untap cost on a permanent or none of it.
 * Effects that last "until end of turn" end during the heal creatures phase.
 * Effects that happen "at end of turn" are processed at the end of the end phase.
 * The active player chooses the order in which "at end of turn" effects are processed. Only one damage prevention step occurs for all "at end of turn" effects.

Activation Costs

 * Where possible, cards with an activation cost are written as “[Cost]: [Effect.] [Activation instructions (if any).]”
 * In general, abilities of permanents that do not include undefined in the activation cost can be used, even when the permanent is tapped; however, this rule does not apply to non-creature artifacts.
 * Normally, when you use an effect, you pay for it once to generate the effect once. However, if you pay the entire cost multiple times simultaneously, multiply the effect that many times. In this case, make all decisions (such as choosing targets) only once for the entire effect. For example, to have your Brothers of Fire deal damage each to two different targets, Pay to have it deal damage to the first target and then another  to have it deal damage to the second. To have it deal 2 damage to a single target, however, you can pay  all at once.

When You Can play a Spell or Use an Ability
If a spell or ability has no target, you can still play it, even if it will have no effect. If a spell or ability requires a target, you can only play it if there is a legal target and only if there are the correct number of legal targets. One exception to these rules is that spells or abilities that prevent or redirect or regenerate creatures can only be played during a damage prevention step when a player or creature has taken damage, or when a creature is on the way to the graveyard, respectively. This is because those spells or abilities target the damage, not the target receiving the damage. The legal targets of a spell or ability are specified in the text of the spell or ability immediately following the word "target”

Resolving Spells and Effects

 * When you resolve a spell or effect, try to complete as much of the effect as possible, except where one part of the effect is a prerequisite for another. For example, consider an effect that says "Tap target creature. Effect deals 1 damage to that creature.” Even if the target creature is already tapped when this effect resolves, the effect will still deal damage to that creature. By contrast, consider an effect that says "Tap target creature to have [effect] deal 1 damage to that creature. If the target creature s already tapped when this effect resolves, the second part of the effect is ignored because the first part of the effect is a prerequisite for the second. Note, however that some spells or effects that appear to have a prerequisite, may not actually have one. The first part of a spell or effect sometimes requires sacrificing a permanent, paying life, or removing counters from a card. These are costs that must be paid when the spell or effect it played (and thus are still played even if the spell or effect is countered or fizzles), rather than prerequisite effects.
 * Except in the case of certain enchantments, after a spell or effect resolves, you do not constantly recheck to see if the target of that spell or effect remains valid; the effect continues to work even if the target becomes illegal after the spell or effect resolves.
 * The characteristics of the source of an effect (for example color, power and toughness, controller, etc.) are determined when the spell or effect is announced. These characteristics can only be modified by interrupts that target the spell or effect.
 * A spell that will not become a permanent goes to the graveyard once it has been resolved or countered; it does not go to the graveyard when played.
 * Triggered effects that are triggered by an event that occurs during the resolution of a spell or effect do not occur in the middle of resolution; rather, they occur immediately after the spell or effect resolves completely.
 * Spells and effects that affect both players at the same time always resolve their effect on the active player first. A player can never lose the game during the resolution of a spell or effect. However, if all players lose the game (other than as a result of being reduced to 0 life or less) during the resolution of a single spell or effect, the game is a draw.

Counter

 * All counters with the same name represent the same type of counter, so any effect that affects a certain type of counter affects all counters of that type, regardless of the source of the counter.

Forced Effects

 * If a card instructs or forces a player do something that player must try to follow the instructions using his or her existing resources (for example, creatures already in play, mana already in that player's mana pool, etc.), but that player is not required to cast spells or use effects to get those sources unless specifically instructed by the cards to do so.

Triggered Effects

 * Triggered effects are effects that only happen or can only be used when a certain event occurs – for example, when a creature is placed in the graveyard. Only one damage prevention step occurs for all triggered effects that are triggered by the resolution of any single spell or effect.

Owner

 * The "Owner" of a card is the player who began the duel with that card in his or her deck. When a card refers to a "caster", it should be read as referring to the "owner", instead. If you use Animate Dead to bring a creature out of someone's graveyard, that player is still the owner of the creature.

Sacrifice
Cards can sacrifice themselves, unless the text specifically says otherwise, as with Lord of the Pit.

Card errata
A few cards had their wording changed:
 * More cards that had said "discard", changed to destroy (for example Tranquility)
 * The rules text on Giant Spider was changed to "Can block creatures with flying" instead of "Does not fly, but can block flying creatures", a much simpler description of what it can do. The rules text on Web was similarly changed from "... can now block flying creatures, though it does not gain [the] flying ability" to simply "... can block creatures with flying."