It may be a little bold to use the definite article, but for my part, here goes:
What do you need to achieve?
When do you need the results?
How much can you spend?
And the supplementary questions:
Why do you need it (to get the business context, or the overall 'mission' you want to conduct)?
Why do you need it then (just what is causing the timing)?
What value do you need to produce (in NPV terms, I'm thinking)?
Once the project is cooking;
What do we need to deliver?
How will we deliver it?
Has this been done before (by the owner, the project manager, the delivery crew...anybody)?
What resources and skills do we need?
Who will do what?
What could stop us?
How will we know when we've finished (that is, what will achieve the required performance?)
Another take on this is the Five Immutable Principles of project management.