For some reason that's much more complicated than forcing data in a main form
Is that because you've always relied on table field properties to enforce data to be entered into each mandatory "main" form field? There's more than one possible approach and which is best/easiest can depend on the nature of the form and your level of db knowledge.
Table relationships is one way. If referential integrity option is used, there has to be a value for at least the linked field in both tables. However, certain user behavior might cause a partial record to be lost as the user manipulates the form.
Another is to validate the required controls upon main form save button click. This can't work if the form is not of the single record type as simply navigating from one record to another in a form with more than one record will attempt to save that record. In other words, a save button is pretty much pointless. You'd have to provide more info about the forms and the process involved if not using relationship method.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.