Whenever you cast an instant or sorcery spell from your hand, you may cast target card with the same name as that spell from your graveyard. (You still pay its costs.)
4/8/2016 You must choose a target for Harness the Storm’s ability (if one exists) immediately after you’ve cast an instant or sorcery spell. There’s no way for that spell to be countered and then targeted by Harness the Storm once it’s in the graveyard.
4/8/2016 You choose whether to cast the targeted card as the ability resolves. You can’t wait to cast it later, and there’s no way for your opponents to set up a situation where you have to cast the card but don’t want to.
4/8/2016 If the spell you cast from your hand is countered in response to Harness the Storm’s triggered ability, you can still cast the target card while the ability is resolving.
4/8/2016 Because you’re paying the spell’s costs, you can pay alternative costs, such as awaken costs and surge costs from the Battle for Zendikar block. You can also pay additional costs, such as kicker costs. If the card has a mandatory additional cost, like Lightning Axe, you must pay it to cast the card.
4/8/2016 The spell you cast from your graveyard can target the spell you cast from your hand.
4/8/2016 The spell you cast from your graveyard resolves before the spell you cast from your hand. It’s put into your graveyard as it resolves.