Kopala, Warden of Waves — ruling 1

To determine the total cost of an opponent's spell that targets a Merfolk you control, start with the mana cost or alternative cost that player is paying, add any cost increases, then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.

Kopala, Warden of Waves — ruling 2

Activated abilities contain a colon. They're generally written "[Cost]: [Effect]."

Kopala, Warden of Waves — ruling 3

Kopala's abilities won't affect triggered abilities (starting with "when," "whenever," or "at") that target a Merfolk you control.

Kopala, Warden of Waves — ruling 4

Spells and abilities that target more than one Merfolk you control cost only {2} more to cast or activate.

Kopala, Warden of Waves : règles, fonctionnement et interactions (MTG)

Kopala, Warden of Waves is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Spells your opponents cast that target a Merfolk you control cost more to cast. Abilities your opponents activate that target a Merfolk you control cost more to activate.

How it works : Kopala, Warden of Waves

Original English text — Wizards of the Coast / Scryfall

#1September 29, 2017· Wizards of the Coast
To determine the total cost of an opponent's spell that targets a Merfolk you control, start with the mana cost or alternative cost that player is paying, add any cost increases, then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.
No other card is named in this ruling.
#2September 29, 2017· Wizards of the Coast
Activated abilities contain a colon. They're generally written "[Cost]: [Effect]."
No other card is named in this ruling.
#3September 29, 2017· Wizards of the Coast
Kopala's abilities won't affect triggered abilities (starting with "when," "whenever," or "at") that target a Merfolk you control.
No other card is named in this ruling.
#4September 29, 2017· Wizards of the Coast
Spells and abilities that target more than one Merfolk you control cost only more to cast or activate.
No other card is named in this ruling.