Are you qualifying the false value? If not, then it will display false, as it is not true. Syntax for if is:
=IF (test, true, false)
If your formula looks like this:
=If (test, b1)
Then it will display false, as you need to set what value to display if it is false, as in:
=If (test, b1, c1-b1)
In the above example, if the result of test is true, then the value of b1 will be displayed. If test is false, then the vakue of c1-b1 will be displayed.