>If I wrote
>
>on error 1881 = messagebox ("bad data")
>
>it works OK, but if I wrote
>
>on error 1881 do form error1
>
>It gives sintax error
>
>What can be wrong ?

You got first thing right by accident only. The correct syntax is:
ON ERROR [your command]
It means you could say:
ON ERROR Do Form Error1
and within the form you could check error number (by using ERROR()) and show appropriate message.