Поле со Списком (ComboBox) - Проверка выбрано ли значение (через ListIndex)
Private Sub txtCourse_AfterUpdate()
If Me!txtCourse.ListIndex = -1 Then
ACourse = 0
Me!txtGroup.Requery
Me!txtGroup.Enabled = False
Else
ACourse = Me!txtCourse
Me!txtGroup.Requery
Me!txtGroup.Enabled = True
End If
End Sub