If you forget to announce a triggered ability, such as Sheoldred's making an opponent sac a creature on their upkeep, does the ability still happen? Does the opponent still have to sac a creature?
It depends of if you actually missed the trigger or just didn’t mention it.
A trigger like Sheoldred’s that doesn’t have a target isn’t missed until the game progresses past the point where the trigger should have resolved. If you remind your opponent to sacrifice a creature as they go to draw a card, then you haven’t missed the trigger.
A player is never responsible for their opponent’s triggers unless they’re controlling that opponent. If the game moves past when a trigger needed to be acknowledged without its controller doing so, then the trigger is missed. In general, if the trigger is mentioned after this point, it’s up to the opponent of the trigger’s controller to choose if they want it to resolve or not.
That’s the basics. Read on if you want to know the more advanced stuff.
When a trigger actually becomes missed varies depending on its effect on the game:
Triggered ablities with one or more targets (other than target opponent in a 2-player game) need to be acknowledged when they’re put on the stack so that appropriate targets can be chosen at that time.
Triggered abilities, like Sheoldred’s sacrifice trigger, that have a visual effect on the game state (e.g. life gain, counters, permanents entering or leaving the battlefield, drawing cards, etc.) but don’t have targets need to be acknowledged when they resolve by the appropriate actions happening.
Triggered abilities that don’t have a visual effect on the game state (e.g. buffing creatures, granting abilities, changing types) don’t need to be acknowledged until their effects matter on the game. (e.g. a creature that was buffed deals damage, or an opponent tries to target a creature that now has hexproof)
There are a couple of classes of trigger that can’t be missed:
Triggered abilities that only set up delayed triggers and triggered abilities that only create copies of a spell on the stack are assumed to resolve automatically. However, the resulting delayed triggers or spells must be acknowledged at the appropriate time.
Triggered abilities with a default action (e.g. “Do X unless you do Y.” or “You may do Y. If you don’t, do X.”) can be missed as normal, but if they’re noticed to be missed then the opponent of that trigger’s controller chooses whether or not the default action (X) happens.
A delayed triggered ability that only causes a permanent to change zones can’t be missed. The opponent of the trigger’s controller chooses if the trigger happens immediately or at the beginning of the next phase if it’s noticed to be missed.
For all other classes of triggered abilities, if they’re noticed to be missed, the opponent of the controller of the triggered ability chooses whether or not the triggered ability will be put on the stack.
