When do effects on cards expire?

Examples:

I use Catachan Outpost on a unit. Does it forget that it has +2ATK if it goes back to my hand? How about if it's destroyed - does it lose the +2ATK?

I use Fortress-Monastery. When does the unit forget that it costs 1 less?

~Andrew

Effects expire when the action is over, when the card says they expire, or when the target leaves play - depending on the individual effect.

Your specific examples:

Catachan Outpost says, "Combat Action: Exhaust this support to give a target unit +2 ATK for its next attack this phase." So once the unit attacks or the phase ends (whichever comes first), the +2 ATK is gone. If the card leaves play before either of those happens, the support's effect ends immediately upon the card leaving play. (See RRG, p. 8: "Card abilities only interact with, and can only target, cards that are in play, unless the ability text specifically refers to an out of play area." - So since Catachan Outpost doesn't say it interacts with cards that are not in play, it immediately ceases to interact with its target if that target leaves play.)

Fortress-Monastery says, "Interrupt: When you deploy a [sPACE MARINE] unit, exhaust this support to reduce that unit’s cost by 1." As an interrupt, this effect modifies the resolution of the deploy action, lowering the cost. Once the deploy action is over, the interrupt's effect is over, too.

I love detailed answers.

How do I tell that the effect of Fortress-Monastery modifies the resolution of the deploy action? To me, it's pretty clear that the only thing it modifies is the cost of the unit. "Reduce the unit's cost by 1" doesn't modify anything other than the cost of the unit.

~Andrew

P.S. ktom - thank you for your detailed, rules-based, and prompt answers. Your efforts are really appreciated by the rules-lawyers among us...

How do I tell that the effect of Fortress-Monastery modifies the resolution of the deploy action?

Because it's an interrupt and that's what it is interrupting.

According to the RRG, p. 9, "Interrupt abilities resolve just before their triggering condition occurs, sometimes canceling or changing the resolution of that condition." To me, this says that, as an interrupt conditioned upon deploying the card, Fortress-Monastery must be modifying the conditions under which the deployment action is being undertaken -- not the base characteristics of the deployed card itself (which would set the conditions of the deployment action on its own if it weren't for the interrupt).

One slight caveat re "once the unit attacks or the phase ends (whichever comes first), the +2 ATK is gone.". If the unit doesn't attack this phase (eg retreats), it loses the +2 ATK at the end of the phase regardless. Which was probably obvious, but just making sure.