hvis en verdi i en tekstboks overstiger et gitt antall, åpnes en meldingsboks(med advarsel eller lignende)

Opps!

If Antal > 30000 Then
msgbox \">30000\", vbInformation
end if

Where you put this depends on a couple of things.

Is the textbox bound to the table? if so then you could put this in the forms before UPDATE event and then you could cancel the update too.

Antal is the field you are checking, this needs to be altered to the correct name.

If Antal > 30000 Then
  msgbox \">30000\", vbInformation
  Cancel = True
end if