Sindarin, in
J.R.R. Tolkien's mythos, was the most spoken Elvish language ever. While originally in his conception the language spoken by the Noldor (second clan of elves, hence the name Noldorin in the Etymologies)
Tolkien later decided it was the language of the Sindar, who had stayed on in Middle-Earth while many of their friends had gone on to Valinor. When the Noldor came back to Middle-Earh, they adopted the Sindarin language, though they believed their native
Quenya to be the more beautiful. Most of the men of Númenor also spoke the language. Tolkien based his Noldorin/Sindarin on Welsh, and indeed, Sindarin has many of the mutations that characterise the Celtic languages.
Sindarin plurals are characterised by "i-affection", as Tolkien calls it. The Sindarin term for this is prestanneth (disturbance, affection) and the English term is "umlaut", a German word used to describe much the same process. What it all comes down to is this: All Sindarin words form their plurals like man/men -- by changing the vowels in the word. The reason for this was that the primitive plural ending "-i" affected the vowels in the word by making them closer to itself. Having done it's dirty work, it disappeared. So Sindarin plurals no longer have "i" at the end, but still have its "residue".
Sindarin has a complex series of mutations. These occur when a short phrase (usually a preposition) occurs before the mutated word, changing its (the mutated word's) first consonant. Many times, the preposition also changes. Also, mutation occurs in many other places (to mention a few, in compounds (elvellyn, from mellyn, "friends") or in direct objects).
Sindarin verbs are also quite complex. There are strong and weak verbs, also called i-stems and a-stems respectively. Just like English (and German) strong and weak verbs, the strong ones are more "irregular" than the weak ones. Sindarin also has quite a large number of irregular verbs.
Sindarin is one of the two languages (the other is Quenya) in which we can write texts that are quite large in size. A lot of information can be found in Helge Fauskanger's Sindarin article, http://www.uib.no/People/hnohf/sindarin.htm. Didier Willis also has an online Sindarin dictionary at http://www.geocities.com/almacq.geo/sindar/. A lot of information can be found on Sindarin from other sources as well.