Q: What happens when a card like The Harrowing instructs me to play another card during its resolution? Do I play it while the other spell or ability is still resolving? What if I can't play it?
To clarify the timing of these and other effects, we're introducing the idea of pending items on the chain. When you play a spell, or use an ability, they will be pending until you finish the steps of playing them before priority passes. When a spell or ability instructs you to play a card or play a token-or when a triggered ability has its trigger condition met-during the playing or resolution of another card or ability, you'll perform the very first step of playing that card or using that ability by putting it on the chain, then stop. This chain item is still pending for now. You'll then process the rest of whatever is currently happening, including putting any other pending items on the chain above it.
Then, in the order the items were put on the chain, each pending item's controller goes through the other steps of putting it on the chain: Make choices, pay costs, and evaluate legality. Only then is the pending item considered to have entered the chain. If it's legal to play and it's a unit or gear, it resolves immediately as usual. Once all pending items are done being fully added to the chain, play continues, and any items that remain on the chain can be reacted to. The most recent items to be placed on the chain will resolve first as usual.
If at any point during that process that player is unable to proceed with playing the pending chain item for any reason, the item is removed from the chain and, if it's a card, is put back where it came from. This won't happen very often-if you start the process of playing a card, you'll almost always be able to legally finish it-but it's a necessary failsafe to handle corner cases.