Cross-posted from G+:
This will probably be complete bollox, but.
We have the following sentence in the Túrin Wrapper Area IV in VT50:
?a?l il cem il chem na en i naugrim en ir Ellath natho thor a den ammen.
(Tolkien's emendations shown as strikethrough.)
Carl theorizes that thor (following the deleted, transparently future copula natho) replaces natho and is an independent future particle with a passive impersonal ending, and den is an adjective related to dínen.
What if thor is not a replacement for natho, but actually this is the adjective/predicate? It could be a variant related to the root TOR-, seen in torech and meaning something like 'hidden, secret, closed'? I cannot readily explain the initial th-, but Tolkien seemed to have a penchant for the liquid mutation in the texts on the Wrapper, perhaps at this time he thought adjectives as predicates underwent a liquid mutation?
That leaves us with den. Elsewhere on the Wrapper we have en as possibly our first attested form of the S present tense copula, which Carl links (probably correctly) to aen from KL, so en: present indicative, aen: subjunctive. What if den is another element in this series, possibly a future one?
As I said, probably incorrect, but food for thought anyway.