How would you write this formula in numbers?

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.

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.

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?