7-9 is the "Yes, but..." result
Rolling 7-9 means a success, but in a way the player didn't fully expect:
On a 7–9, you stumble, hesitate, or flinch: the GM will offer you a worse outcome, hard bargain, or ugly choice.
The key difference is — does the player have any choice or not:
- hard bargain — "yes you can do it, but you have to do X beforehand" or "you can do it, but it will lead to Y" (the player can choose to pay the price)
- ugly choice — "yes you did it, now you must choose between X and Y" (the player must choose one of two bad things)
- worse outcome — "yes you did it, but now you suffer the following consequences" (the easiest option — no choice here, just make a GM move)
The "worse outcome" result is a success with complications, or a partial success — the PC does what he/she wanted to do —, but with unexpected (usually bad) consequences, fully dictated by the GM.
There is no "Nothing happened" result in DW
Any Move result — being success, failure, or both — should move the story forward. Here's a quote from Read and Understand Dungeon World:
something happens, something besides just failure. Instead of being a dead end, a player's failure leads to consequences: the situation gets worse or they have to pay a price.
Read the whole document for more details about DW being different from other TRPGs.