Like all Elvish languages, the origin of Sindarin dates back to the Common Quenderin [CQ] language spoken by the Elves when they awoke beside the shores of Cuiviénen (S. Nen Echui). Even in those early days, the Elves divided themselves into three tribes, and Sindarin is ultimately derived from the dialect of the third tribe, later called the Teleri.
When the Valar discovered the Elves, they invited them to come live in Aman. Some of the Elves refused and stayed behind, becoming the Avari. But many agreed, including all of the first tribe and many of the second and third tribes, collectively called the Eldar. This time of journey is called the Common Eldarin period [CE], but even in this early period the third tribe lagged behind the other Eldar, which is the origin of their name (Teleri means “Hindmost”). It is likely that the Telerin dialect already started to diverge from that of the other Eldar, even in the Common Eldarin period.
The Teleri were the last to arrive to the western shores of Middle Earth, and when they arrived, their leader Elwë met and fell in love with the Maia Melian, and was lost to his people. Not wanting to abandon Elwë, the Telerin people remained on the shores of Middle Earth while the first and second tribes departed for Valinor. But after more than a century it became clear that Elwë would not soon be found and the third tribe split in two. Many Teleri departed for Aman but others stayed behind in Beleriand, thereby becoming the Sindar (Grey Elves) so called because they stopped short of seeing the light of Aman. The time where the Teleri were separated from the first and second tribes but had not yet themselves split in two is called the Ancient Telerin [AT] period, and this ancient dialect is the common ancestor of both the modern Telerin language and Sindarin.
After the departure of the Amanya Teleri (the Telerin branch that went to Aman), the Sindarin language evolved separately from all other Elvish languages, in a period referred to as Old Sindarin [OS]. In this period, Elwë was finally found again, and he took up leadership of the Sindar under the name Thingol. The exact boundaries of Old Sindarin are not clear, but it almost certainly predates the return of Morgoth and the arrival of the Noldorin exiles in Middle Earth.
The time immediately after the rise of the Sun and the wars with Morgoth was period of unusually swift change for the Sindarin language. The chaos and many displacements of the Elvish peoples led to an intermingling of dialects and linguistic evolution that was nearly as rapid as the evolution of human languages. Normally the Elvish languages changed more slowly than human languages, and the many deaths from the wars with Morgoth is perhaps the only time where the language of the immortal elves underwent “generational” changes like that of humans. Despite the many changes of this period, Tolkien did not further subdivide it, lumping it all into a broad “Sindarin” period.
After the fall of Beleriand, Sindarin entered into a period of somewhat surprising stability. This “Late Sindarin” period spans the entirety of the Second and Third Ages of Middle Earth, over six millennia in all. Despite this vast period of time, the Sindarin language at the end of the Third Age was only slightly different from that spoken at the end of the First Age. My pet theory is that the relative changelessness of Sindarin in the Second and Third Ages was the result of the stabilizing influence of the three Elven Rings, but Tolkien himself never addressed this specific question. In this period, Sindarin gradually displaced the Nandorin tongue that was originally spoken by the Elves of western Middle Earth, though Nandorin may have survived in places like Mirkwood.
Tolkien gave a rough timeline of the early history of the Elves in the Annals of Aman (MR/48-134), written in the 1950s. The dating system in the Annals of Aman used “Years of the Trees” [YT], since they describe the time period before the rise of the Sun and the Moon. Each “Year of the Tree” marked roughly 9.5 solar years of time (MR/60), so the periods described are much longer that they first appear. Also note that Tolkien continued to work on his histories through 1960s, and this chronology may not completely reflect his later conception of the histories. With those caveats, the major periods of Sindarin’s historical development seem to be:
- Common Quenderin [CQ]: From the awakening of the Elves (YT 1050) through the First Sundering (YT 1105): approximately 500 solar years.
- Common Eldarin [CE]: Lasted until the Vanyar and Noldor crossing the sea to Valinor leaving the Teleri behind (YT 1132): roughly 250 solar years.
- Ancient Telerin [AT]: Up through the departure of the Amanya Teleri (YT 1150): roughly 170 solar years.
- Old Sindarin [OS]: From the departure of the Amanya Teleri to the fall of the Two Trees and the rise of the Sun and Moon (YT 1500): roughly 3300 solar years.
- Sindarin [S]: Spanning the Years of the Sun until the end of the First Age: roughly 600 solar years.
- Late Sindarin: The Second and Third Ages of Middle Earth: roughly 6500 solar years (the three Elven Rings being present for roughly the last 4000 years).
As noted above, the precise boundary between “Old Sindarin” and “Sindarin” isn’t clear, so the respective length of those two periods of evolution is uncertain. David Salo introduced a “Middle Sindarin” period between Old Sindarin and more modern Sindarin in his book Gateway to Sindarin (GS/60), which makes it easier to organize the many phonological changes in the history of Sindarin. However, Tolkien himself didn’t use this term and any assignment of Middle Sindarin to a specific period of time would be a wild guess, so I don’t use it here.