An IF works by evaluating a logic situation, then returning one answer
if the logic is evaluated as True and another if the evaluation is
False. The format is IF(logical question, true evaluation, false
evaluation).

So if I read your post right, your formula should look like this:
=IF(B840,B8-40,0)
This returns a zero if B8 is less than 40. You can enter another
response such as "" (2 double quotes) if you want to return a blank
cell.

Does that do it for you?
Dave O