I am just learning Dungeon World, and I don't understand how to prevent the characters from just trying impossible things. For example, what if a character tries to jump across a 1,000-foot-wide gorge?
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It's impossible by the method given, so it doesn't happen (…yet). Dungeon World's rules allow any action to be attempted — not any goal. If a goal is declared without describing how it's accomplished, the group is not ready to pull out the dice to see what happens next and if the action achieves the goal, because what's happening now (the action or actions) hasn't even been mentioned.
But let's back up, and see where this essential truth about the game comes from.
The rules are conversation rules, not a physics
The thing to remember about Dungeon World is that the rules are not a “physics” for the world. They are instead rules about who is allowed to say what and when during your shared discussion about an imaginary story. And given how often it's the GM's turn to say something, the large percentage of the game's rules that appear in the GM chapters are central to how the game is designed to work.
When the players say they do something, one of two things happen:
When a player says that their PC does something impossible, you already know it's impossible. That triggers a GM move — specifically, the trigger is “Whenever everyone looks to you [the GM] to see what happens”; Dungeon World, “The GM”, “Moves”, page 165 — which you then use to choose your conversational response to the declaration.
You can choose any GM move that would be in line with the GM's Principles (p. 162) and Agenda (p. 161), but the one most relevant here is probably Turn their move back on them: basically, ask them “Cool, ok. How do you do that?”
When they answer, they will hopefully give you a reasonable, believable action that can accomplish the desired goal. When they do, that either happens (and you make a GM move to describe the result) or a PC move's trigger is matched and then that must be followed.
If though, when they answer, they respond with “but I'm allowed to try anything!” your response should be “Yes, and that's fine, but you're not trying anything, you're just saying what you want to accomplish. If you tell me what you do to try to get to the other side, then I can tell you what happens next.”*
For example, if their action is literally only “I just jump across” and their character has no obvious means of super-human jumping, your Principles and Agenda demand that you use the GM move Tell them the requirements or consequences and ask:
You tell them the consequence of their action, and you ask if that's what they want to do. If they say yes? Well then, they're plummeting into the chasm! Your GM Agenda and Principles will again kick in here and demand you make a GM move, and probably something other than the instant death of Fighter happens, because that's the least interesting thing that could to happen next. However, the result will probably be a hard move (p. 166) of some kind with interesting but irrevocable consequences.
* This interaction here isn't part of the conversation rules. Since you're not discussing the fictional world, those rules aren't involved. Here, what's happening is you've all stepped back from the fictional world and are, for a moment, discussing the game rules themselves. This won't happen very often once everyone gets the hang of the game.
Actions, not goals
So the strong degree of agency that Dungeon World gives players is not about giving them permission to achieve any goal they want, just by rolling the dice once. The agency is to let players attempt any action they want, and have their attempt respected by the world responding in an interesting and believable way.
By stringing together actions, they can work towards their goals. Trying to put together actions that will bring them closer to their goals — and dealing with the complications when their actions don't succeed immediately — is what makes being a Dungeon World player interesting and powerful-feeling.
You make it a couple of yards out from the edge and then begin plummeting towards the jagged rocks below. What do you do?
Now perhaps the character has access to a move that can get them out of the situation (such as a spell, artefact, or wings) but the most likely thing is that they flail their arms wildly and crash to the earth, in which case…
Now that is a bit harsh, so you'd probably want to open with something more along the lines of:
All else being equal, the distance you can jump is going to be about 990 feet too short to make it across, so you're going to be killing yourself unless you can think of an out. Are you sure you want to do that?
… it is always possible that that there was a breakdown in communications and the player isn't actually aware that you said the gorge was wider than a man can jump.
In short: Follow the fiction. Actions have consequences.
Normally (I mentioned some possible exceptions earlier), leaping across a chasm will kill a character. It isn't defying danger (or any other basic move); it is simply leaping into certain death.