A starting point for explaining this would be that you only roll in challenging situations. If the stakes are low, then a move isn't triggered.

So rather than having a "can I open the door" dice roll, we assume that if there is no particular danger there then we can just walk up and open the door. There might be a trap there, of course, in which case your thief could roll Tricks Of The Trade to find out ( risking failure ) or the person opening the door might need to Defy Danger when the trap is triggered. Perhaps the door is locked and the Fighter might choose to roll Bend Bars Lift Gates to bust through it. All of these are moves that have definite stakes and consequences to failure, so you roll them straight.

If things get difficult then what changes is not that individual rolls necessarily become more challenging ( although that is at your discretion as a GM ) but instead that more moves are triggered. So if you want to run across an empty room, no moves are going to be triggered, if you want to run across a room full of angry armed goblins then you're going to have to make some fancy moves to arrive at the other side safely - you're sure to need to Defy Danger a time or two, probably in different ways, maybe you'll have to Hack And Slash your way past them. Every time you do, your chances of hitting a failure increase and when you fail ( or even hit an intermediate success ) then things start to happen that impact your plan. Maybe your sword has broken, maybe one of the Goblins runs off with the treasure you came here for in the first place, perhaps they have a cave troll!

These failures and the interplay between failure and success are what drives the story ( and consequently the action ) in Dungeon World. A failed roll isn't "you can't do it," it is "now something bad is going to happen and you are going to have to decide what to do about it."

A closing point to explaining this might be that it's not really a question of how difficult something is, it is a question of how many ways it goes wrong. The rules aren't telling your GM to think awkward, it is their job to think dangerous.

A starting point for explaining this could be that you only roll in challenging situations. A move is triggered when the players make a decision to act in a way that will have interesting consequences whether they succeed or fail. And no matter how calm things seem right now, an outright failure is going to result in the GM making a hard move, which can quickly begin a slide into mayhem.

So rather than having a "can I open the door" dice roll, we assume that if there is no particular danger there then we can just walk up and open the door. There might be a trap there, of course, in which case your thief could decide to search for one, triggering the Tricks Of The Trade ( risking failure ) or the person opening the door might need to react when the trap is triggered and Defy Danger. Perhaps the door is locked and the Fighter might choose to bust through it, triggering Bend Bars Lift Gates. All of these are moves that have definite stakes and consequences to failure, so you roll them straight.

If things get difficult then what changes is not that individual rolls necessarily become more challenging ( although that is at your discretion as a GM ) but instead that more moves are triggered. So if you want to run across an empty room, no moves are going to be triggered, if you want to run across a room full of angry armed goblins then you're going to have to make some fancy moves to arrive at the other side safely - you're sure to need to Defy Danger a time or two, probably in different ways, maybe you'll have to Hack And Slash your way past them. Every time you do, your chances of hitting a failure increase and when you fail ( or even hit an intermediate success ) then things start to happen that impact your plan. Maybe your sword has broken, maybe one of the Goblins runs off with the treasure you came here for in the first place, perhaps they have a cave troll!

These failures and the interplay between failure and success are what drives the story ( and consequently the action ) in Dungeon World. A failed roll isn't "you can't do it," it is "now something bad is going to happen and you are going to have to decide what to do about it."

A closing point to explaining this might be that it's not really a question of how difficult something is, it is a question of how many ways it goes wrong. The rules aren't telling your GM to think awkward, it is their job to think dangerous.