Plural patterns in Sindarin
I apologize for how complicated it is to make plurals in Sindarin, I wish it were easier. (At the bottom of this post there is also a table if you find that more convenient. I would recommend still reading over the list, because it still includes some important information. As well as examples of course.)
Originally, Sindarin had three noun forms; singular, dual, and plural. Luckily the dual quickly became obsolete except in writing. Instead there is an additional class plural, which coexists with the regular plural. I will not bore you with the dual form. The class plural will get its own post.
Anyhow. The final vowels in a word change to make a plural. If a word is monosyllabic, that is the final vowel.
For example: orod (=mountain), lass (=leaf)
However, non-final syllables can also change. I’ll list the final syllabic mutations first:
A becomes ai.
This one is very well attested in Tolkiens writings:
tâl (=foot) -> tail
cant (=shape) -> caint
rach (=wagon/wain) -> raich
E becomes i, long ê becomes long î.
As seen in the attested forms:
edhel (=elf) -> edhil
ereg (=holly tree) -> erig
Laegel (=green elf) -> Laegil
certh (=rune) -> cirth
telch (=stem) -> tilch
hên (=child) -> hîn
têw (=letter) -> tîw
I becomes i, it does not change.
O becomes y, long ó becomes long ý.
As seen in:
orch (=orc) -> yrch
toll (=island) -> tyll
bór (=trusty man) -> býr
U becomes y, long û becomes ui
With this one, it does not matter wether it‘s a final syllable or not.
tulus (=poplar) -> tylys
dûr (=dark) -> duir
Y becomes y, it does not change in plural.
Any singular was likely a plural as well, ylf (=drinking vessel) was likely both the singular as well as the plural. (A little like how sheep is both the singular and the plural in English.)
Au becomes oe, this one is not as well attested as the previous ones.
In Noldorin a similar pattern was seen, where au became ui, but in Sindarin there are many excepions to this rule.
raw (=lion) -> roe
Other dipthongs do not change in the plural. Ai is an exception.
Ai becomes i, usually î, more rarely ý. Wether the ai changes in a word has to do with the word origin, if it originated as a Quenya word ending in -ya, it changes. If not, it does not change in the plural form. I recommend using Eldamo to research the word origins.
fair (=mortal) -> fîr (originated as firya in Quenya)
cai (=fence) -> cî
cair (=ship) -> cîr
gwain (=new) -> gwîn
lhain (=thin/meager) -> lhîn
mail (=dear) -> mîl
paich (=juice/syrup) -> pich (note the short i)
Now for the non-final syllabic mutations. In plural forms, the non-final syllable can also change! Aren’t you thrilled.
A becomes e
As seen in several well attested forms:
barad (=tower) -> beraid
lavan (=animal) -> levain
aran (=king) -> erain
amon (=hill) -> emyn
annon (=great gate) -> ennyn
lalven (=elm-tree) -> lelvin
malen (=yellow) -> melin
E is unchanged in non-final syllables.
I (i) is unchanged.
O normally becomes e, there are exceptions; if it originated from ancient ā it does not change.
Golodh (=Noldo) -> Gelydh
onod (=ent) -> enyd
Rodon (=Vala) -> Rodyn
U becomes y.
Y stays unchanged.
For clarity’s sake, I have also made this table. I hope this helps, as I realize the list format is not super easy to search through.