A colleague asked about a sentence such as “What additional information would you need in order to determine if XYZ will actually happen?” Should the will also be would?
The answer is that it depends. Is the possibility of XYZ happening also contingent or hypothetical? If it’s something that may or may not happen regardless of whether you make a determination in advance, then “will” is preferable:
If you were a weatherman, what information would you need in order to determine whether it will be cloudy tomorrow?
On the other hand, if XYZ’s occurrence is hypothetical, then “would” is correct:
If you were obsessed with a star, what information would you need to determine if he/she would accept your proposal of marriage?
It’s possible to have a hypothetical with bearing on a real event, so we can’t insist on concord between the conditionals without looking at the sense.
Incidentally, some people will insist that you should always shorten in order to to plain to. In fact, while there are places where the shortening can be accomplished to good effect, there are others where bare to would be ambiguous:
These are the dishes I need in order to cook. [Without these casseroles and plates, I can’t cook.]
These are the dishes I need to cook. [I need to cook these dishes.]
And how about if versus whether? While whether is more formal and has no possible ambiguity, if is very well established in such usage, and has been used by far better authors than the ones who will tut-tut you for using it. Again, consider tone and clarity.