The Gnomish Lexicon of the 1910s had a demonstrative pronoun G. en glossed “that by you, that already mentioned (by you), that past” (GL/32). In an untranslated sentence appearing in the Early Noldorin Grammar (EGQ) of the 1920s, it seems ᴱN. en might be used as a conjunction “then”: manthil sóg odog en maint os eneg bhair might mean something like “having eaten he drinks a lot, then he ate around six houses” according to Gilson, Welden, Hostetter and Wynne (PE13/128 and note #78).

Neo-Sindarin: For purposes of Neo-Sindarin, I would retain ᴺS. en as an adverb or conjuction meaning “then, [G.] that past”, in keeping with its demonstrative use in ennas “there” from around 1950 (SD/129). In Demonstrative, Relative, and Correlative Stems (DRC) from 1948 Tolkien said “In Q[uenya] ... but not in all Eldarin languages it [en] is (in time) restricted to the future and always refers forward in time” (PE23/97). This means that in other Eldarin languages like Sindarin, it is possible that en could be used temporally to refer to the past. I would only use en for “then” and not as a demonstrative “that”, for which I would use ᴺS. sa instead.