Like English, Sindarin has a definite article i “the”, but unlike English it has no indefinite article “a, an”. In Sindarin, an indefinite item is specified by the bare noun: “the man” = i adan but “a man” = adan. The definite article is effectively a proclitic, closely associated with the following word, and as a result causes soft mutation.
Sindarin has a number vowel mutations that serve various grammatical functions. The best known is i-affection, which plays a major role in Sindarin plural nouns and adjectives, but is a factor in the conjugation of the Sindarin present tense as well. I divide i-affection into 3 different “flavors” of changes: internal i-affection, final i-affection and final i-intrusion.
Sibilant mutation results from an ancient preceding s that was caused various mutation effects before being lost. The two best examples of sibilant mutation are the preposition o “about” and (possibly) the conjunction a “and”. The most complete description of sibilant mutation appears in a discussion of one of the etymologies of “and”:
Liquid mutation results from a preceding liquid r, l. This is a rather speculative mutation, based only a couple of examples and what we know about Sindarin’s phonetic history. The only examples of liquid mutation appear in a couple of phrases from a document referred to as the Túrin Wrapper (VT50/5):
Stop mutation results from a preceding (ancient) voiced or voiceless stop t and d, generally from a preceding preposition. This stop is typically lost before consonants with various mutational effects. This is a somewhat speculative mutation, since Tolkien described it but we have no actual examples of this mutation in an attested sentences, except for in one rejected note where Tolkien considered making na-chaered stop mutation rather than soft mutation (PE17/147, where he described it as being from nat- or nad-).
One interesting feature of Sindarin is that its nasal mutations are distinct from those of Welsh. In particular, the nasal mutations of p, t, c are ph, th, ch rather than voiceless nasals “mh, nh, ngh” as in Welsh. This is especially peculiar given that Sindarin mostly underwent the same medial phonetic developments as Welsh, whereby (for example) nt > nth > nnh (long voiceless nasal) > nn.