Triggers can trigger at any time, but wait to go on the stack until a player would get priority.

I cast while you control . What happens? After a creature spell resolves, a player receives priority. But it's not as simple as that. Before priority can be received, first the game has to check for state-based actions (SBAs) - things like creatures with lethal damage dying, or +1/+1 and -1/-1 counters canceling each other out. Then after it makes a check where no SBAs are performed, the game checks for triggered abilities that have triggered since the last time a player received priority, and

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Can you respond to the choice of what to copy with Phantasmal Image? It depends!

This is a very common question with any kind of copy creature. The most important thing to note is that the static ability that lets you choose to copy something is NOT a triggered ability (if it was, pretty much every Clone-type creature would die for being 0/0 before you could choose to copy something!). It's a replacement effect that modifies how it enters the battlefield. So what does that mean for you? If you let a resolve, the choice to copy something is made immediately as it resolves

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