Do triggered abilities from 'on play' effects (like Darius or Ravenbloom Student) trigger when you cast a spell or when it resolves, and what happens if the spell is countered?

Ruling: Cards are "played" when they resolve, not when they are cast. Triggered abilities from "on play" effects trigger after the spell fully resolves. If a spell is countered (e.g., by Defy), it does not resolve and is not considered played, so it will not trigger any "on play" abilities.

Sequence:

• Cast a spell (put it on the chain)
• Spell resolves
• The spell is now considered "played"
• Any "on play" triggered abilities go on the stack
• If the chain was empty, this creates a new chain for the triggered ability
• Players get priority to respond to the triggered ability

Nuances:

• Whether the trigger is on the same chain or a new chain doesn't affect reaction windows - you get a reaction window either way
• The term "playing a card" refers to both the act of putting it on the chain and its resolution, but "played" status only occurs upon resolution
• Unless something says to do it "as you play", effects happen after resolution