When 'de' with an adjective
I made a list of how I imagine it is done, can someone please comment on and correct it?
En ny bok - de ny boken - nya böcker - de nya böckerna.
Ett gammalt brev - de gammalt brevet - gammla brev - de gammla brev.
Yes, the paradigm is helpful to show the logic of the language, though eventually you should find that you do not need it any more. Then, a sentence is correct just because it sounds comfortable to you. It cannot be easily reached, but given efforts, it will happen some day. :)