IF Functions on adobe form

Assuming that you want this to be a custom calculation script for a field, it could be something like:

// Custom calculation script for text field

(function () {

    // Get the field values, as numbers

    var B21 = +getField("B21").value;

    var B22 = +getField("B22").value;

    // Set this field's value

    event.value = B21 > B22 ? B21 - B22 : 0;

})();

Change "B21" and "B22" in the getField statements with the actual names of the fields in your form.

Acrobat forms use JavaScript and not Excel functions. There is no if function on JavaScript

There is an if statement whose syntax is very different.

Have looked at any JavaScript tutorials or Acrobat scripting tutorials?