When discussing Tolkien's works in detail, one will inevitably have to use a number of Elvish terms. Sometimes it is necessary to know both the singular and plural forms of a word to be able to use it correctly.
The section What common mistakes should I try to avoid? in the Tolkien Newsgroups FAQ explains why it is good to know a little about Elvish plurals, and lists some of the most commonly confused forms. This document is a bit more detailed overview of the plural and singular forms of Tolkien's Elvish terms. It could, I hope, be useful as a reference to readers of Tolkien who are uncertain of the correct form of some word. Also, if this little FAQ were to guide someone to the fascinating world of Tolkienian linguistics, I would be very pleased.
Here are some of the commonly used Elvish terms, as well as some rarer ones. I have tried to check which of these forms are actually attested in Tolkien's published texts, and the words I haven't been able to verify (or haven't checked yet) are marked with an asterisk (*). However, I think it is safe to assume that these, too, are quite regular. Variant forms and spellings are noted in parentheses.
|kasar (casar)||kasari or kasári|
|nauko (*nauco)||naukor (*naucor)|
|noldo (ñoldo)||noldor (ñoldor)|
|*úmanya (*úamanya)||úmanyar (úamanyar)|
|valarauko (*valarauco)||valaraukar (*valaraucar) (!)|
|nogoth niben||noegyth nibin|
With Quenya, things are quite simple. If the singular ends in a consonant, add -i to form the plural (teler : teleri); if it ends in a vowel other than e, add -r (elda : eldar); and if the final vowel is e, replace it with i (quendë : quendi). (An exception to the last rule: -ië becomes -iër in the plural.) Sometimes the stem of the word is slightly different in the plural, though this is rare. The most familiar example is silmaril, which takes the form silmarill- before endings, so that its plural becomes silmarilli (and the plural genitive silmarillion). The Ardalambion Quenya article gives much more information about the structure of the language.
Sindarin plurals are a bit more complicated. They are normally formed by changing the vowels of the word (orch : yrch). Back vowels in the singular usually become front vowels in the plural. This phenomenon is called i-affection, since it was caused by the ancient Elvish plural ending -î, which itself later disappeared in Sindarin. Sindarin also has a class plural, meaning "all of a group" (e.g. nogothrim). To learn more of Grey-elven plural formation, see the Ardalambion Sindarin article. There is also a list of attested Sindarin plurals at the Mellonath Daeron website.
In general, just do as Tolkien did in the books. With some of the words listed above, both the Elvish and the English plural can be used. Tolkien usually wrote "Silmarils" rather that "Silmarilli", and it should be quite appropriate to say, for example, "talans" and "mallorns", though I can't find examples (in The Lord of the Rings, Tolkien used "mallorn-trees" and "mellyrn"). With most of his Quenya terms Tolkien seems to have used only the original Elvish plural. Still, it wouldn't seem terribly wrong to me to say "there are a few tehtas missing in this text" or "some fëas refused the summons to Mandos", even though tehtar and fëar are preferred.
Notes on spelling: