If I am wielding a long spear against a goblin with a dagger ("Staying far out of it's reach, I make quick thrusts at the goblin's head!"), do I roll Hack and Slash? Do I simply roll damage? What happens if I roll Hack and Slash and roll an 8?
You roll hack and slash as long as, narratively, it makes sense for the goblin to be at the spear's weapon range. If the goblin is already on top of you, you can't use hack and slash to damage the goblin with the spear.
The 7-9 result, "the enemy makes an attack against you" could be a monster move or a GM move.
Basic Moves: Hack and Slash
The enemy’s counterattack can be any GM move made directly with that creature. A goblin might just attack you back, or they might jam a poisoned needle into your veins. Life’s tough, isn’t it?
Monster Setting 1, Cavern Dwellers: Goblin
- Charge!
- Call more goblins
- Retreat and return with (many) more
Charging you with its dagger could also be reasonable, since "Charge" is a goblin move. The fact that you're trying to stay out of its reach is being compromised by its move and your middling roll. Dealing damage probably isn't appropriate, but the goblin could get inside your spear range (sign of an approaching threat, show off a downside to their equipment).
And if I have a spiked shield (hand weapon) and want to attack a large ogre with a halberd (reach weapon), do I have to Defy Danger to get up close, but then get to hit automatically once I'm close?
It's a defy danger to get close because the danger is an ogre with a reach weapon, without which you can't hack and slash. But, having defied the danger, you still have to make a hack and slash roll if you want to jab the ogre.
One of the writers of Dungeon World, Sage Latorra, addressed this in this thread.
A note on shapeshifting in general: taking on a new form is, in a way, saving up successes for particular tasks. The druid makes one roll and, through the clever selection of a form, can turn that into 1 or more successes. The cost of this is the new form: since everything is triggered by the fiction taking on a new form changes what rules engage. That's the fundamental tradeoff of a new form: that form's strengths turn your one roll into more successes, but that form also limits your options.
So it would seem that this move was indeed intended to be used as automatic future successes. The druid could also make a Hack & Slash roll without spending 1 Hold, though.
Best Answer
That makes perfect sense. Remember though: Hack and Slash is not just that one strike, it's the synopsis of a fight. But if you want a particular moment to stand out, because it's truly epic (like a difficult boss finally going down, for instance) you could use the move just for this single strike.
Maybe the player had to get close to the monster to deliver the killing blow, and the monster got the chance to claw and bite while it was being gutted? Or the player lost control of his/her weapon while striking, dislocating a shoulder in the progress? Or the dead monster fell on top of the player, its spiny exoskeleton piercing the player's leather clothing? Or the monster coughed up a ball of bile with its last breath and the acid burned the player's face?
It makes perfect sense. But remember to keep the fiction flowing afterward, don't just deal damage: Is the player now disabled in some way? Is he pinned or blinded? And what's next?