Subform to populate text field on main form.

I am just leaning access so detailed steps to the code would be helpful.  

Basically I have a text field "txt1" that populates my main table with numbers that either enter or take off two subforms "subform1" &"subform2" depending on the response that i need to give. I have queried  the subforms to look up the info that i need based on a "combobox1"  

Is there a code for when i add a new line to either subform1 or subform2 to have my text field auto populate with the number i entered in.  There will never be a case when on the same record i would enter a number into subform1 and subform2. there are two  fields in my subforms the first one is the new numbers I want to populate the text box and the second is the parent field that sets the criteria this field is hidden.  

Any help or advice would be appreciated! Thank You!