How to include the only fields which were modified on the form with the before/after field values in the email body

You have two options. One option is to use a native email notification with default settings. Then QuickBase will natively tell you which fields changed and only the fields that changed.

The other option which is not quite what you want is that in an email notification you can you syntax like this

The status changed from [old.status] to [status]. 

That will tell you the old value and the new value but you have no way of only outputting text for what changed. So for all the fields that did not change the old status on the new status will be the same.