For abilities with 'you may' wording (like Tideturner), when do you decide whether to use the ability - when it triggers or when it resolves? Can the opponent react to it?

Ruling: When an ability with "you may" wording triggers, you must choose targets during activation (before opponent can react), but you decide whether to actually execute the "you may" effect during resolution.

Sequence:

• The ability triggers and goes onto the chain if there is a valid target
• You choose any required targets as part of activation
• Opponent can now react to the ability on the chain
• When the ability resolves, you decide whether to use the "you may" option

Nuances:

• This ruling comes from Riot devs and Chinese judges, though it has been acknowledged as confusing and may be clarified in upcoming rules updates
• If there are no valid targets, the ability does not trigger at all