It sounds like you're trying to do something like this:
'Assumes this is fired by the onOK event of an editform
Dim theEditForm
Set theEditForm = ThisEvent.Object 'gets a reference to the edit form.
theEditForm.Pages("pagename").controls("control_one").value = theEditForm.Fields("field_name").value
'So, for example, you could now concatenate the values of a couple of fields, and write them to a combo or edit box as you see fit.
Dim temp
temp = theEditForm.Fields("field_name_one").value & " " & theEditForm.Fields("field_name_two").value
theEditForm.Pages("pagename").controls("control_one").value = temp
'or
theEditForm.Fields("other field name").value = temp
The things in the Customizing ArcPad help that are most useful to me are the pages titled "<something> object", for instance "form object" or "page object". The examples are sort of buried, so I can understand your frustration with that, but the "object" pages have some examples of getting references to these objects and thier basic use. Hope this helps!