So everybody knows that Tolkien was an expert on languages, right? That he made up languages for the elves and everyone else in Middle Earth and rather than have them speaking English like he had to write them doing ANYWAY, because no one would read the damn books, he came up with Sindarin & FinnishQuenya,but if you don't speak Quenya or Finnish, it REALLY looks like Finnish because why not two languages? Okay, but since he's a much better linguist than a geographer and doesn't want to have to think up more than one elf country, he had to come up with a reason for there to be two elf languages (but Gonder & Dale & Bree all speak the same language). And the reason for that, is the divisions of the elves at the dawn of the First Age, before anyone was keeping track of Ages in Middle Earth. See, first they divided into the Eldar & the Avarri, the former traveling west into Beleriand and beyond to Tirion, Alqualonde & Eressea in Aman. Then they divided along the way, with some Eldar staying east of the Misty Mountains and some stopping in Beleriand and some passing over the great sea to Aman. And because of this geographical estrangement, their languages changed. The Noldor, Vanyar & Falmari Teleri kept speaking Quenya, although there is a line that says the Falmari's speech changed on Eressea before they were able to cross the sea and build Alqualonde. But anyway, over in Aman, they still spoke Quenya, which since their own name for Elves was "Quendi" meaning people who speak, I assume Quenya means speech or soemthing like that. Meanwhile, back in Beleriand, the Elves become known as the Sindar, the Gray Elves, because their king is Elwe Singollo or Elu Thingol (because languages), and their speech becomes known as Sindarin. And then the Noldor come back to Beleriand and we get the two different languages, and somehow, the Noldor get really good at speaking Sindarin and so that becomes the lingua franca of the elves (they also use it when speaking to the Dwarves, because they have trouble learning Dwarven and the Dwarves think it's easier to learn Sindarin than to teach Elves to speak Dwarf - because that makes sense, that Middle Earth's master race and Firstborn Children of Iluvatar can't learn another language but Dwarves can. ANYWAY, eventually, Thingol finds out why the Noldor came back and is pissed and so he outlaws Quenya. No one is allowed to speak Quenya if they are under his rule or want to have any dealings with his people, and the Noldor, all proud and haughty and in this fix because they don't take no for an answer are just fine with that and start using Sindarin except for their rulers among themselves for like Elf ceremonies I guess? So Quenya falls into disuse and becomes what Tolkien himself calls Elf Latin.

And then Finrod, who is the nicest lord of the Noldor and kinsman to Thingol and still in his good graces, for a Noldo, so presumably he made the switch to Quenya, finds Men. A whole new species of people! Now he's one of the Calaquendi, who has seen the Light of the Two Trees in Valinor and so he's about as special as a person born in Arda can be, so it's fairly easy for him to learn Man speech in one sitting and he can sort of read their minds anyway. And the Men learn Elf to talk to Elves, even eventually using it among themselves. They have a head start in learning it, because on their way to Beleriand, they met Avarri and learned some bits of Elf speech from them. Remeber the Avarri - the ones who stayed behind way back before all the elves split up? How the Udun would Avarri learn Sindar? They spoke Quenya, just like all the Elves did at first, before the Sindarin tongue evolved in western Beleriand (simultaneously perhaps, among the Falathrim and the Nandor and the Elves of Eglador? ), so how did they help Men get a jump on learning Sindarin?

But the REAL issue is that the origin of Sindarin does not even make sense! The story is all very plausible, because it's akin to what happened in the real world with humans and human languages, which is no doubt what inspired Tolkien in his creations. But these are ELVES we are talking about. They are immortal. Thingol grew up speaking Quenya, along with his buddy Finwe, who was the King of the Noldor in Valinor. Finwe & Thingol journeyed together as representatives for their race to Valinor to see the home the Valar were offering to them before returning and leading their peoples west into Valinor and Beleriand, respectively. Then Finwe was murdered in Valinor and his two eldest sons led all his grandkids back to Middle Earth where they butted heads with Thingol over their past crimes and the whole language issue. Galadriel and Finrod were Finwe's grandkids. Even if they were out of touch for thousands of years, it's only two or three generations for elves. Supposedly it was difficult for the Sindarin to learn Quenya, but their leaders, Thingol and Cirdan, and Thingol's wife Melian, all lived before Sindarin was even a thing! Also, Thingol's brother, who was Galadriel's father-in-law if I am remembering it right and the grandather of Nimloth, who married Thingol's grandson and gave birth to Elrond's mother. See what I mean? There are only a few elves in between the dawn of their very race and the elves who appeared in the movies which chronicle the end of the Third Age. They waited a long time to have kids. Luthien was over three thousand years old when she met Beren (and they died like 30 years later - talk about coming to an abrupt stop), and from everything we know about elves, was probably a virgin. She certainly didn't have any kids. IIRC, Feanor was the only Elf (aside from his wife) to have seven kids. They just don't do it that much, so you can't even say "Well, that's just the rulers, there are probably lots of elf generations begetting and whatnot under their rule."

So Quenya changed to Sindarin more or less during the lifetime of the people who spoke it. Am I missing something here?