It doesn't look like there's very much Quenya activity going on here - this looks more like a place where lame spam goes to die. I'm very new to Quenya myself, but if you don't mind I'd like to take a crack at your sentence. My understanding of Quenya grammar is based on the Quenya course written by Helge Fauskanger (HF), and his online dictioary, so if you're coming at this from a different angle, your mileage may vary.
Here's what I came up with:
Nái nauvas calina lyen írë ilyë exë vanyuvar.
Literally, "May it be that it will be [a] light for you when all others will disappear."
Here are some reasons why my translation differs from the one you suggested:
1) "nái nauvas" instead of "nai": My "nauvas" is the future of the verb "to be", with the suffix "-s" representing the subject pronoun "it". The way HF describes "nái", it's an impersonal construction that turns the following future-tense sentence into a wish. One way to say this in English is "may it be that ...", but the "it" in this phrase is just a dummy subject that stands in for the following "that ..." clause. It's not a reference to a particular thing, the way "it" is in your English sentence. I could be wrong, but I don't think there are any examples where "nái" means anything like "may [some particular thing] be [some other particular thing]".
2) "calina" instead of "alca": According to HF's dictionary, "alca" means "ray of light", and it doesn't make sense to me to speak of rays of light going out. It's lights that go out, and HF gives "calina" as "a light". This is abstracted from the word "coacalina", which means "the light of the house" ("coa" = "house") and is a metaphor for the soul. I think this is reasonably close to the kind of light you're talking about.
3) "lyen" instead of "lyenna": "A light to you" strikes me as more a dative notion ("to or for you") than an allative one ("towards you"). This is probably just a matter of taste.
4) "se mornie nome": I couldn't figure out what this means ("mornie" = "darkness"?), but it doesn't seem to reflect anything in the English version of the sentence.
5) "ilyë exë" instead of "fare exa alca": Based on HF's dictionary, think your version says "sufficiency other ray of light". Mine, I think, says "all (plural) other ones". I don't know if you can use the plural of "other" this way in Quenya, to mean "other ones", but you could always substitute "ilyë exë calinar" ("all other lights") if you wanted.
6) "vanyuvar" instead of "vanya arse". I've made my verb future, because this appears to be how Tolkien did it. For example, Fíriel's song has "írë Anarinya queluva", which is translated as "when my sun faileth", but is literally "when my sun will fail". English is lax about using the present in subordinate clauses to describe events that are strictly future, but other languages (Latin, for example, and apparently Quenya) are less forgiving. "Vanyuvar" is plural to agree with my plural "ilyë exë [calinar]". I don't quite understand what "arse" means. This shows up in HF's dictionary as "he is out"; are you using it for "out"? If so, it's not necessary, because the verb "vanya-" by itself means "disappear, go away", and it seems to me that this is sufficient to express the idea of a light "going out". Other verbs are possible: "firuvar" ("will fade"), for example, and even "queluvar", as in Fíriel's song - although for some reason the corresponding verb stem ("quel-"? "quela-"?) isn't listed in HF's dictionary.
I'll repeat that I'm very new to Quenya, and I could be entirely wrong about some or all of this. I've jumped in here only because it didn't seem likely that anyone else was going to.
Best of luck,
--- In firstname.lastname@example.org, "mich7s" <mich7s@...> wrote:
> my first attempt to translate...
> Nai alca lyenna se mornie nome ire fare exa alca vanya arse.
> May it be a light to you when all others go out.
> it's probably horrendouse :( please help!