I just woke up, therefore some of the syntax here might not be the best. Without further a-do:
I would definitely not say "Ni hí an-nathad", nor "ni hí an-eithad". That is an Anglo-based idea of "to see", "to hear", etc. but what about Tolkien's "E aníra tírad"? It most definitely is not "E aníra an-dírad" and I will not pretend that it is. The only way that I'd feel comfortable with such a form is if we accept the "ir" in "ir Ithil ammen Eruchîn" to be the "ir" in "aníra". That would make the sentence mean "The Moon longs for us", simply taking the "an-" and putting it onto the plural first-person pronoun "we". But as it doesn't say "îr", as one might expect from "tûl acharn" and "gûren bêd enni", and this is an often unthoughtof translation, it might not be the case. Thus, the "an-" before verbs is most likely a reconstruction that should not be tolerated. The gerund functions itself as the "to" before the verb. Examples: "aníron istad" (I want to know), "e cên i iest dín gwannad eb ethûl i Aran" (he sees her desire to leave before the King comes back). Additionally, as the only attested form of the locative "sí" comes syntactically attached to a verb "linnathon", we have to follow this. Yes, we have "adar nín i vi Menel", with no sign of "na-" anywhere, but that is because it is reasonably assumed. While the "sí" in "ni hí" is clearly lenited because of the "na-" and I credit you for that, we must follow the form of "linnathon...sí". Thus, "non hí" should be our guess. Non? Why not naun? I'm going off of "aun" (he/She/it gave), which is why "I gave" is Ónen, an attested form. I would thus say "Non hí nathad/eithad/eliad (basically "non hí" plus the gerund form of whatever we want to use for "help"). This sentence structure resembles another Tolkien sentence "Edro hi ammen", a syntactical parallelism which I love. Going on from √THĀ/√ATHA, David Salo said "Telin le thaed" for a very similar sentence. Perhaps we should consider "thaed"? I personally prefer "natha-". On that note, nam hí le nathad!