To add validation rules to a form, you open the form in Layout view and add rules to the properties of individual controls.
For more information about data types, field sizes, and input masks, see the article Introduction to data types and field properties.
You can define validation rules for table fields and for controls on forms.
This is the same as setting the Required field property to Yes.
However, when you enable the Required property and a user fails to enter a value, Access displays a somewhat unfriendly error message.
The steps in the section Add a validation rule to a control on a form, later in this article, explain how to add rules to the properties in individual controls.