Abilities that create replacement effects, such as a permanent entering the battlefield tapped or with counters on it, can’t be targeted. Abilities that apply “as [this creature] enters the battlefield” are also replacement effects and can’t be targeted. Similarly, the replacement effect that prevents a creature with a stun counter on it from untapping can’t be countered.

Activated abilities are written in the form “Cost: Effect.” Some keyword abilities, such as equip and crew, are activated abilities and will have colons in their reminder texts.

If you counter a delayed triggered ability that triggers at the beginning of the “next” occurrence of a specified step or phase, that ability won’t trigger again the following time that phase or step occurs.

Triggered abilities use the word “when,” “whenever,” or “at.” They’re often written as “[Trigger condition], [effect].” Some keyword abilities, such as prowess and fabricate, are triggered abilities and will have “when,” “whenever,” or “at” in their reminder text.