How to link SubForm to Multiple Keys Main Form?

I'm sorry but i am a noob in navision and i am not sure for the right way to ask..

As you can see the form above, the Store No., Item No., Year and Month(from subform) are primary keys.
What kind of setting should i establish so every time after i input Store No., Item No., and Year, the subform below will change accordingly for me to input data?

Yup, same table