Field validation based on text value in other field
I'm a new user, but can't figure out this basic field validation -- in one case I need to require the entry of an 18 number value in field B if (and only if) field A has a "yes" value from a pull down box. In the other case, I require a date in field D if (and only if) field C has a "yes" value from a pull down box. I've searched and cannot find anything that provides me with the validation logic for these cases -- I'm sure its pretty basic. I"m assuming that a custom error message - in both cases - can be used from the box at the bottom of the Validation Options dialog box. Thanks in advance for any help!