What about vánes rénenyallo for 'I forgot it'?.

+Björn Fromén that could work

This is case where I think I would use the early G. laitha- "forget" as the basis for a neologism rather than coining a new circumlocution.

The root LITH does not reappear in Tolkien's later writing, though the similar root LIT does appear as the basis for Q litse, S lith "sand, ash". We can further postulate that LITH survived only in its a-fortified form laith- to better distinguish it from the derivatives of LIT. Thus Q. laisa-, S. laitha- "to let slip, lose, mislay, forget". Perhaps in Quenya it became associated with the etymologically unrelated laista "ignorance".

Note that the early root LIÞI seems to have something to do with speed (G. lint "quick"), and might have been modified to the (rejected?) root (S)LIT appearing on PE22/127. It could therefore be distantly related to Q. linta "swift", though I can't see a way to derive linta from LITH, especially given its cognate S lim "swift".

+Paul Strack I wonder what the primary meaning of the root LITHI is. "Pass"? Passing of time? Akin to later SKEY?

+Tamas Ferencz That sounds plausible to me.

+Paul Strack I like the idea of using GL root laithá-, but I am not sure about its phonetic development.

+Tamas Ferencz +Paul Strack +Ицхак Пензев It seems to me that G. laitha- actually survived in S. as leitha-. In fact, I've been using it as "to forget" for some time now. And I don't think that Neo-S. laitha- works well, since an A-infixed "i" þundóma would rather yield laetha-, which assumes that A-infixion for verbs survived past Noldorin (albeit we have aur < √UR), along with the root LITH. Additionally, HoME-III/154 offers √LETH and its derivation leithia- which is later appointed to √LEK. Thus, if we deem it unwise to stretch the meaning of leith(i)a- and Q. lehta-, perhaps we can use the partially attested leg- (cf. adleg-) in S. and lec- in Q. instead.

+Ekin Gören I was only considering the Quenya form. I agree that if G. laitha were adapted so Sindarin it would have to become leitha. I prefer using that over assigning this meaning to leithia-, but if one is opposed to using early forms, this seems like a decent compromise. It’s also possible the two forms were blended in Sindarin while remaining distinct in Quenya.

I know only little Sindarin, but these arguments make sense. I'd vote for lec-.

I have just realized that at one point I created the neologism *etsen/esten-, it's in VQP:
middangeard.org.uk - nq:etsen [VinQuettaParma]

+Tamas Ferencz I'm not sure if it fits "forget":
eldamo.org - Eldamo : Quenya : etelehta-

+Ekin Gören I think that verb is too much connected to its attested gloss; but if you think of it, my proposed etsen/esten uses the same concept just with a different root

A different root which shares the same meaning (i.e. LEK = SEN). Combined with the same concept, "out+release", it is certainly more evocative of "rescue" than "forget". So etelehta- & "esten-" would be synonymous, like urya- & "narya-".