If an opponent plays a card with an optional on-enter ability ("When I'm played, you may do x") and chooses not to do the effect, is there still an opportunity to react?

Ruling: Yes, you can react to the triggered ability when it is placed on the chain, before your opponent makes the decision about the "may" effect.

Sequence:

• The card is played and the trigger condition is met
• The triggered ability is placed on the chain (this is when you get priority to react)
• If no reactions are played, the ability resolves and the opponent makes their decision about the "may" effect

Nuances:

• All triggers in this game are mandatory - the ability always triggers regardless of whether the "may" effect will be used
• The "may" section denotes decisions made on resolution, not whether the ability triggers
• Once the opponent has already made the decision for the "may" section, the ability is already resolving and there's no window to react