Sindarin is a very hard language since it had changed so many times from is archaic form to the one that is used in the Third Age. For this section, the old form is really important to understand to be able to write it correctly. +This grammar section is not 100 % sure, but this is what the expert had work out and came to.+

One important change that occurred in the evolution of Sindarin was that final vowels were lost. Hence an old word like ndakro "battle" later became ndakr. In early Sindarin, this word appeared as dagr. Another example is makla "sword" later appearing as makl, early Sindarin magl. We must assume that the plural of words like dagr, magl was formed after the same pattern as other monosyllables of comparable shape, like alph "swan", pl. eilph. So the plurals "battles" and "swords" would presumably be deigr, meigl (this would be before ei in a final syllable normally become ai).

What complicates matters is that words like dagr and magl were eventually changed. The final r, l came to constitute a separate syllable, so that for instance magl was pronounced mag-l just like English "eagle" is pronounced eeg-l. Later, these syllabic consonants turned into full-fledged normal syllables as a vowel o developed before them: Dagr (dag-r) turned into dagor and magl (mag-l) became magol. (Incidentally, the latter word was apparently often replaced by megil, which must be an adapted form of the Quenya word for "sword", namely macil.) The plurals deigr, meigl would presumably undergo the same process to become deigor, meigol (and the late change ei > ai in final syllables would never occur simply because ei was no longer in the final syllable). From a synchronic point of view, this results in what looks like irregularities: Normally, singular words like dagor and magol would be expected to have plural forms degyr, megyl, since o in the final syllable normally becomes y in the plural (e.g. amon "hill" vs. emyn "hills"). But in cases like dagor or magol, the o intruded relatively late and seems to be younger than the umlaut o > y; hence such newly developed o's would - presumably - remain untouched by the umlaut. If Tolkien did not imagine that analogical levelling bulldozed these "irregularities" out of existence, all two-syllable words where the second syllable contains a secondarily developed o must still be treated as monosyllables as far as plural formation is concerned. The o must be left alone and the vowel in the "second-to-last" syllable must be treated as if it were the vowel in the final syllable, which is precisely what it used to be.

The adjectives and nouns in question are:

badhor "judge" (pl. beidhor if the theory holds - otherwise it would be analogical bedhyr),
bragol "sudden, violent" (pl. breigol; this adjective also appears as bregol, pl. presumably brigol),
dagor "battle" (pl. deigor),
glamor "echo" (pl. gleimor),
hador "thrower, hurler" (pl. heidor),
hathol "axe" (pl. heithol),
idhor "thoughtfulness" (unchanged in the pl.; luckily a noun with this meaning normally will not require a pl. form),
ivor?"Crystal" (unchanged in the pl.),
lagor "swift" (pl. leigor),
maethor "warrior" (unchanged in the pl.),
magol "sword" (pl. meigol),
magor "swordsman" (pl. meigor),
nadhor "pasture" (pl. neidhor),
nagol "tooth" (pl. neigol),
naugol "dwarf" (pl. noegol),
tadol "double" (pl. teidol),
tathor "willow" (pl. teithor),
tavor "knocker, woodpecker" (pl. teivor),
tegol "pen" (pl. tigol)
Perhaps gollor "magician" also belongs on this list (pl. gyllor rather than? Gellyr).

From the Ardalambion, The Noble tongue, (Monosyllable becoming Polysyllabel)