Sindarin plurals change from the inside, instead of having an S tacked on the end. It's difficult at first, but remember that once upon a time, in the distant past, English also made plurals by changing the vowels inside them.

Remember: when nouns become plural, their adjectives also become plural.

Accents

But first, a quick note about the circumflex and acute accents. When a word gets more syllables, its circumflex accent becomes an acute accent and it's acute accent disappears.
Examples:
West + man: Dûn + adan = Dúnadan
Grey + pilgrim: Mith + randír = Mithrandir

Regular Plurals

The trick to Sindarin plurals is knowing what type of vowels you are changing: final or non-final.

Non-final vowels are all of the vowels in a word except the last vowel, or if the word is only one syllable long, there is no non-final vowels in it. Here is a list of how all of the non-final vowels that change.
A-->E
O-->E
U-->Y
AU-->OE
AW-->OE

Examples:
Coron (globe) - Ceryn (globes)
Bauglir (oppressor) - Boeglir (oppressors)
Ganlenas (tobacco plant) - Genlenais (tobacco plants)

Final vowels are the last vowel in a word if it isn't a U without any consonants following it [This is because in the word's history, the U used to be a W.] and if the word is only one syllable long.
A-->AI
E-->I
IE-->I
O-->Y
IO-->Y
U-->Y
Ú/Û-->UI
AU-->OE
AW-->OE

All of the accents are kept through the transition, unless the transition is into a diphthong. So, an Ê becomes an Î, and so on and so forth.

Examples:
Hên (child) - Hîn (children)
Gwaun (goose) - Gwoen (geese)
Galu (good fortune) - Gelu (good fortunes)
Gûr (heart) - Guir (hearts)
Firieth (mortal maiden) - Firith (mortal maidens)

Class Plurals

There are two plurals that do entail tacking a suffix on, and these are called "Class Plurals." Class plurals are used on nouns. If they are added to the end of an adjective, say "green" for example, then it would mean "green ones." This is important to remember when pluralizing adjectives with nouns, because the meaning of the phrase could be drastically altered with hilarious effects. When a noun is a class plural, it's adjective is a normal plural.

The first class plural is made by adding "-rim" to the end of a word. It is the equivalent to putting "a group of," "some", "a collection of", or "a bunch of" before a word in English. One odd little rule about using it: if the last letter of the word being pluralized is an L, then "-lim" is suffixed on instead of "-rim".

Examples:
Gobel (town) - Gobellim (Some towns)
Randír (wanderer) - Randirrim (A group of wanderers)

The second class plural is made by adding "-ath" to the end of a word. It is the equivalent of putting "all of the" or "all" before a word in English. It also has a little oddity to its usage. If the last letter of the word is an M that came from MB or an N that came from ND, two N's or M's precede it.

Examples:
Glîn (gleam) - Glínnath (All of the gleams)
Nem (nose) - Nemmath (All noses)
Paur (hand) - Paurath (All hands)

Special Plurals

There are a few exceptions, and they are marked in (good) dictionaries. These exceptions are remnants of grammar no longer in use in the Sindarin language.

The dual singular. This is for denoting one of a couple, pair, or set of twins.
Lhewig (ear) - Lhaw (ears)
Gwanunig (twin) - Gwanûn (twins)

The host of evil. Used only for military regiments of mistrusted and feared things, it is an old equivalent to "-rim". It is related to the -hoth of counting.
Daur (werewolf) - Daurhoth (a host or regiment of werewolves)
Dorn (dwarf) - Dornhoth (a host or regiment of dwarves)
Glam (noisy speech) - Glamhoth (a noisy host)

"From" "With" "And" "About"

Elves like their words to flow. So, after some common words ending in vowels, they add consonants when the word was preceding a vowel to make the word flow easily into the next. The words are:
A(r) - and
A(h) - with
O(d) - from
O(h) - about/concerning

Examples:
Aragorn and Arwen - Aragorn ar Arwen.
A bird and a song - Aew a glîr.

Conjunctions

In English, we have many conjunctions. For, and, nor, but, or, yet, so. In Sindarin, there is only three: "and" (a[r]), "or" (egor), and "but" (dan).
"For", "and", and "so" can be translated with (a[r]).
"But" and "yet" can be translated with (dan).
"Or" and "nor" can be translated with (egor).
In a list, the conjunction is only used between the last two items of the list.

Homework

Symbols that you may need:
áéíóúý
ÁÉÍÓÚÝ
âêîôûŷ
ÂÊÎÔÛŶ

Wordbank

Nouns:
Aur-day
Caul-affliction
Caw-top
Coth-enemy
Firion-mortal man
Lim(b)-fish
Nûr-race
Rammas-great wall
Tawar-forest
Tulus-poplar tree
Úthaes-temptation

Adjectives:
Forgam-right-handed
Iphant-old
Naru-red
Níniel-tearful
Sauthannen-drained

For this assignment, don't worry about Lenition, I chose words that it doesn't apply to.

Make plural all of the words in the word bank.

Aur - oer
Caul - coel
Caw - coe
Coth - cyth
Firion - firyn
Lim - lim
Nûr - nŷr
Rammas - remmais
Tawar - toeair
Tulus - tylys
Úthaes - ýthaes
Forgam - fergaim
Iphant - iphaint
Naru - neru
Níniel - nínil
Sauthannen - soethennin

Make all of the nouns class plurals.

Au - Aurrim
Cau - Caullim
Caw - Cawrim
Coth - Cothrim
Firion - Firionrim
Lim - Limrim
Nûr - Núrrim
Rammas - Rammasrim
Tawar - Tawarrim
Tulus - Tulusrim
Úthaes - Uthaesrim
Aur - Aurath
Caul - Caulath
Caw - Cawath
Coth - Cothath
Firion - Firionath
Lim - Limmath
Nûr - Núrath
Rammas - Rammasath
Tawar - Tawarath
Tulus - Tulusath
Úthaes - Uthaesath

Translate the following into Sindarin:

All right-handed fish are old. - Limmath fergaim iphaint.
Old mortal men with temptations are fish. - Firyn iphaint ah ýthaes lim.

Into English:

Uthaesrim iphaint o lim firyn. - Some old temptations, concerning fish, are mortal men.
Limmath a firyn ýthaes. - All fish and mortal men are temptations.