Yesterday, our keeper struggled with the vessel’s move : A thinning of the veil.
Since he’s supposed to not know the explanation to any of the mysteries surrounding the threat until the players do the ‘Answer a Question move’. He had some trouble figuring how to answer “What is about to happen?” and “Why is this about to happen?”without spoiling a later ‘Answer a Question’ move.
What are your thoughts on that matter? 
For context, here is the relevant rule excerpt: "A THINNING OF THE VEIL: When you take this move, choose your divination tools and add them to your Personal Quarters:
…
When you consult your divination tools to glimpse the future, roll with Sensitivity. On a hit, you may ask “What is about to happen?” Keeper will answer truthfully but not necessarily completely. On a 10+, you may also ask “Why is this about to happen?” On a 12+, you also find a Clue; the Keeper will describe it.
…”
First, I think the Keeper can suggest things with what and why they are about to happen without defining the mystery or answer to the mystery. The Keeper is already suggesting and defining things about the mystery by selecting which clues are found and where. It is still ultimately up to the players to decide what is a red haring and what is actually going on.
But I think that, rather than answering “what is about to happen” or “why” about the mystery when this move is invoked, the Keeper might be better served by focusing on anything the vessel would “glimpse in the future” per the move. This is an opportunity for the Keeper to forecast what is about to happen with a particular NPC, or, if the hunters are a split party, what is about to happen in another scene with a different hunter. It may also be an opportunity to suggest what is about to happen with the Vessel and the Vessel’s interactions with darker entities.
In any case, I think the Keeper may find it helpful to keep the descriptions short and maybe even somewhat vague. What do you see?“You see someone, maybe even the American, being attacked with an oversized knife!” “You see a blue sapphire coming near to Hargrave House” “You see a dark figure throwing a glowing object into the bushes outside the house on St. James Street.” Why is this about to happen? “because the American is getting to close the the Figgs and Obert Figg wants to get him out of the way” “because the Mastermind is going to pay the house a visit and invite a hunter on a date” “because someone is trying to hide a Clue.”
These would be easy futures for the Keeper to deliver on probably within the next session or two, and they haven’t defined the mystery. A lot will depend on the context of play though. Hope some of that is helpful!
Delafren’s advice is good! I’d add that as Keeper, I often do know pretty specific things that are about to happen that I can use here. For example, the last time the Vessel in my game used this move, I’d already decided to introduce the Mastermind clue ‘an invitation to a ball at Braithewaite Hall’, so, ‘you sense that you’re about to receive an invitation’ was a natural choice.
Another time, the hunters had rolled a 6- to Answer A Question on the St James St Ghost, so when the Vessel Thinned The Veil I told him, your planned ritual to move the ghost on is going to fail. Why? Because there’s an important clue you’ve overlooked. (Of course, there wasn’t an important clue before that moment, but I followed my principles and the fiction to interpret the results of the move.)
Also, when I run The Between I try to ‘cut’ between hunters pretty often. If I knew one of the other hunters had just rolled a miss, I might outline the reaction I was about to make, giving the Vessel an opportunity to intervene to prevent it.
While there’s no definitive solution until the table answers the question, everyone at the table will have a view of who/what might be going on, so I view this as an opportunity to bounce the question back at the Vessel to shape the narrative in a particular direction. ‘I think the [insert NPC] is about to leave [insert Location] and disappear in a cab’ is the Vessel saying they think that character needs looking in to and might precipitate an opportunity to do so.
That might generates a clue and the fiction moves forward.
Thanks a lot everyone! 