How would you write this formula in numbers?

I agree with BadUnit.. the formula works as-is, including copy/pasted into a spreadsheet.

Are you saying that the formula doesn't work for some reason?

What do you get? an unexpected value? An error message?

Exactly the same way. If it is not working, the only thing that looks odd is the quotes being ” not ". It works fine when copy/pasted from here to Numbers, though.

For the most part (for better or worse) Excel formulas and Numbers formulas are cross-compatible. It makes it far easier for users to migrate to Numbers from Excel, and there's a huge saving for Apple not having to reinvent the wheel (discussion whether or not that wheel needs re-inventing I'll leave for a different day :)

It probably looks different because of how Numbers highlights referenced cells in the spreadsheet:

For any referenced cell, Numbers shows the reference (e.g 'F3' or 'F4' in this example) in a colored oval to help distinguish it from plain text, or a function name. It also highlights the cell itself in the same color to give a visual clue as to where the formula is referencing.

If you copy the formula and paste it into a text-based application, you get the 'plain' version: =IF(F3="","",F3-F4), so it's mostly just a visual thing.