if the form is bound to data, there will be an event on the form before data is updated where data validation can be done.
Go to the design view of the form.
turn on the Property Sheet if it is not showing by pressing Alt-Enter
The Property Sheet shows additional information about what is selected.
Click in the upper left where the rulers intersect to select the form.
On the Property Sheet, click on the Event tab, and then in the BeforeUpdate event.
Click on the DropDown arrow and choose [Event Procedure] from the list.
Then click on the Builder Button to the right ...
for now, just enter this line between the Sub procedure declaration and End Sub:
- MsgBox "This is the form Before Update event"
once you have that much, we can ask you questions to help you construct the code you will need. You should see this message when you try to save the record after changing something by moving to the next record or closing the form or pressing Ctrl-S to save.