hold an action
when you cast a spell you are supposed to be able to respond to it yourself, you are supposed to be able to hold priority.
example, i was in a game of standard playing a deck based around ivy, gleeful spellthief. i had a venerated rotpriest and ivy in play
i cast a enchantment of venerated rotpriest and then used ivys trigger to copy it over to ivy
my opponent cut downed my venerated rotpriest which i also copied to ivy
you see i had a tyvars stand in my hand which i could’ve used to protect them both and by copying the spell i would’ve giving them an extra poison counter
but what instead happened was that i couldn’t play anything until my ivy died and the next time i could respond was to the original cut down