Can you counter a triggered ability with Stifle in the same way you counter an activated ability?

Stifle reads: "Counter target activated or triggered ability." Both activated abilities (written as Cost: Effect) and triggered abilities (written as 'When,' 'Whenever,' or 'At') go on the stack and can be responded to, just like spells. Stifle counters either type with equal ease.

When a triggered ability triggers, it is placed on the stack the next time a player would receive priority (CR 603.3). Once it is on the stack, it is a legal target for Stifle. Countering it with Stifle removes it from the stack and it has no effect (CR 509.2, 603.6).

It is important to note that Stifle cannot target spells or static abilities — only activated or triggered abilities. Mana abilities also cannot be targeted because they don't use the stack (CR 605.3b).

Example: Your opponent attacks with Sheoldred, the Whispering One, and its triggered ability triggers at the beginning of their upkeep to reanimate a creature from their graveyard. In response to that triggered ability sitting on the stack, you cast Stifle. The trigger is countered and no creature is returned.

So whether the ability is activated (e.g., a planeswalker's +1 ability) or triggered (e.g., 'When this enters the battlefield…'), Stifle handles both the same way once they are on the stack.