Hi Carlos!
I just started learning JabRef and was struggling with the exact same problem.
Quick answer:
I just found this solution by inspecting the library file and reading JabRefs documentation about using a free-form search expression.
By selecting a read status, a parameter is added to your entry in the .bib file which will look like this:
@InProceedings{keyword,
author = {...},
title = {...},
year = {...},
readstatus = {skimmed},
}
The status will be either “skimmed” or “read”. If nothing is selected, the parameter doesn’t exist.
So to create a free-form search, where every unread entry is included, you have to filter entries that do NOT have the status “skimmed” AND do NOT have the status “read”:
not readstatus=read and not readstatus=skimmed
I hope this is useful to you