Sychronised Combo Boxes And Filling In Forms

Hi All,
Totally new to access, been asked to make a database for a historical society. To make life easy, in descibing the hist. society's numerous items, I thought it best to follow the standard museum catalogue naming system, i.e everything has a primary and secondary classification. So what I have done is create two combo boxes, with the choices in the 'secondary' classification field limited by the choice in the 'primary' combo box. These controls are based on two tables that contain only the names/info on primary and secondary classification. It works nicely. However, because these controls are on a data entry form, when you make your choices in these combo boxes, the data is not recorded on the appropriate data table (presumably as the controls are linked to those other tables). Everything else you type in the form (like the item's location or autonumber) is saved in correct table.
Other problem is that every time you go to add a new record, and change the primary/secondary choices, it changes all the records on the form to equal these choices.
How do I fix it, or is there nothing I can do (as it may be totally impossible - I don't know!)??

Any help greatly appreciated, but please talk very simply and slowly - I am a complete novice!