I decided to separate the plural presthanneth in two parts, it make it easier to understand then in one parts., so we have seen the vowel, and now we will see the diphthong.

In the Sindarin there is group of letter that we call diphthong, These group will form a particular plural.

The diphthong AU/AW: turns into oe in the plural

        Raw ------­> roe

There is exception with some words, for they came from an older Sindarin that was call Ilkori, in this we can see thau=thôn (pine)

*Ilkori= this was the beginning of the sindarin tongues, so it is very old, older then the Noldorin*.

Also, we can see au change to o in a normal singular form, if it is placed on a second element of a compound and is unstressed. The best example I can think of is Balraug that we see in the form of Balrog but it will still stick to the rules of au to oe so the plural of Blarog will be balroeg not Balryg, like a normal o do in plural.

        Raug=demon
        Balraug---Balrog----Balroeg

We have to know where the name come from at first , in this case the etymologie.

Other diphthongs:

For the other diphthong, (ae, ai, ei, ui) we miss example to confirm it, but following what has been done before we can say that it is probably the way they would react.

So for this diphthong, ae-ai-ei-ui: they do not change in plural.

** There is an exception for AI **

There is some categories of words, that aiwill changed to î but this is rare, This is a list of words that we know of:

        Fair (mortal)---------------------> Fîr
        Cair (ship) ---------------------> Cîr
        Cai (fence) ---------------------> Cî
        Gwain (new)---------------------> Gwîn
        Lhain (lean, thin, meager)-----> Lhîn
        Mail (dear) ---------------------> Mîl
        Paich (juice)---------------------> Pîch

When ai is on the first compound of an element it change to i: Cirdan (shipwright) but it don’t change in the final element of a compound, Narwain: nar (sun) –wain (new) =new sun

There is other exception where ai change to y, when ai represent ei from even older oï (etymologies), the plural should be y the words in question are:

        Fair (adj right, noun right-hand)-------Fyr
        Rain (slot, spoor, tract, footprint)------Ryn
        Tellain (sole of foot)--------tellyn

*Yes you read correctly, Fair can mean Mortal or right (hand), so this is why we have to be careful about the etymologies of words*