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After Scout's Warning resolves, you can suspend a creature card in your hand any time you can cast an instant. As soon as you actually cast a creature card, you lose this capability.

If you cast multiple Scout's Warnings on the same turn, they'll all apply to the very next creature spell you cast.

You cannot use this effect to play Dryad Arbor during an opponent's turn.

You don't choose a creature card when Scout's Warning resolves. Rather, this sets up a rule that is true for you until the turn ends or until you cast a creature card, even if you cast that creature at a time you normally could.