Can you play creatures during upkeep?

Assuming it is your turn, you can play any type of spell or ability during this phase. By contrast, since it’s your turn, opponents can only play instants or abilities at this time (not creatures, sorceries, etc).

Can you respond to a tap ability?

You can’t respond to “being tapped”; you can only respond to spells being cast and abilities being activated or triggered.

Can you interrupt a tap ability?

The ability itself can be responded to, so it can be stopped by doing something like countering the ability or making the target illegal in response.

What happens at the beginning of enchanted creature’s upkeep?

At the beginning of the upkeep of enchanted creature’s controller, put a -0/-1 counter on that creature. When enchanted creature dies, that creature’s controller chooses a creature that Takklemaggot could enchant. If the player does, return Takklemaggot to the battlefield under your control attached to that creature.

What happens at the beginning of an opponent’s upkeep?

At the beginning of each opponent’s upkeep, you manifest the top card of that player’s library. (Put it onto the battlefield face down as a 2/2 creature. Turn it face up any time for its mana cost if it’s a creature card.) Whenever a creature you control but don’t own dies, its owner loses 2 life and you gain 2 life.

What happens at the beginning of your next upkeep?

At the beginning of your next upkeep, you may cast this card from exile without paying its mana cost.) At the beginning of the upkeep of enchanted creature’s controller, put a -0/-1 counter on that creature. When enchanted creature dies, that creature’s controller chooses a creature that Takklemaggot could enchant.

When is an ability not a triggered ability?

That’s it, really. If the ability doesn’t have one of those three words, it’s not a triggered ability. Note, however, that some keyword abilities, such as Flanking, are triggered abilities, even though those particular words don’t appear on the face of the card.