William,
You can use Javascript to validate or VFP. Javascript will validate on the client while VFP validates on the server. An example of Javascript validation can be found in the demo:
http://www.activevfp.com/avfpdemo2/default.aspx?action=detail&cust_id=RICAR
Look at the html source to see how the Javascript works. To see it in action, blank out the e-mail field and press submit...

>Hello Claude...
>
>I would like to validate a form with personal details to
>trap "back-send","back-send"... user action.
>
>The user fill some fields. i need to lock false replies.
>
>Your help is great appreciated!
>
>att.
>William