access class microsoft training - Access

Hi David,
Thank you for your reply. Unfortunately it does not answer my question.
Please remember that I have done a beginners course. Therefore my knowledge is very limited.
What is a macro? This was not covered on the day.
What I don't understand is if I have a primary or index key already in the database why, when trying to save a form do I get the error message of " A primary or index number cannot be a null value"?
To use that well worn movie quote "Treat me like I'm a 3 year old" !
Look forward to your reply.
Regards
Dave Willis