[RPG] How does dungeon world combat work?

First off, all of edgerunner's answers are great. But I wanted to add some Dungeon World specifics:

Check p.19 and you'll see that 6- isn't "failure" - it's "trouble". The GM will say what happens and the player will mark XP. You are attaching non-DW simulationist ideas to DW mechanics by your supposition that 6- means "failure."

These principles can apply in all sorts of games, and have been used by GMs for years. If the PCs have to climb a fence, they're just going to keep trying until they succeed, right? So even in traditional games, many GMs will read "failed" rolls as a lack of some quality - not fast enough, not quietly enough, not without hurting themselves, etc., instead of just keeping them on the wrong side of the fence.

This is because failure is boring and stops moving the story forward. So you are correct, there is no plain-old failure in DW. It's not in the GM's agenda to make the PCs fail. There is no move for failure.

So the problem isn't that edgerunner's ideas are non-optimal, it's that your concept of what 6- means is wrong and that static failure doesn't exist in Dungeon World.

Expanding on 6-

From the text:

Generally when the players are just looking at you to find out what happens you make a soft move, otherwise you make a hard move.

Somewhere in Apocalypse World itself it says about hard moves:

make as hard and direct a move as you like

Early PbtA games like DW assumed you understood Apocalypse World. And this phrase is often tacitly implied in PbtA games even today.

6- means trouble as I said. The GM is free, on 6-, to make a move as hard as they like. That doesn't mean as hard as you can think of.

AW says:

It’s not the meaner the better, although mean is often good. Best is: make it irrevocable.

So while a 7-9 should substantially give the character what they wanted (they accomplish their intent even if their action created complication), on 6- you are free to deny the intent (the action still has to have consequences beyond "no" though) and in addition make a move as hard and direct and irrevocable as you like.

Climbing a mountain a soft move is "The boulders above you on the rock face begin to wobble as the grappling hook you've tossed up there sets itself. What do you do?"

A harder move is "The boulders have tumbled off the edge of the ledge and after hanging nearly motionless for a tiny instant above you, are now plummeting towards you, gaining speed every moment. What do you do?"

A really hard move is "The boulders are yanked free by your grappling hook and come smashing into you, tearing you from your narrow perch and scattering the contents of your pack into the yawning emptiness beneath. What do you do?"

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.