system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we know the verbal pronunciations of words and phrases from dead languages?

I've been playing a lot of civilization 6, and one of the cool things it does is have the characters speak the language of their Civ, even if nobody speaks it anymore, like Sarmatian, for example. It got me wondering about how we know the pronunciation of those words even though we've never actually heard a native speaker speak it. Thoughts?
gpt
Basically it just involves a ton of extrapolation through related languages that still exist, or through the modern descendants of those languages. For instance, we know how ancient Egyptian sounded because of modern Coptic, which is the religious/ceremonial language for Christians in Egypt, and the only living descendant language of ancient Egyptian. 

If it's a language with a ton of writing, as in the case of Latin and Greek, there's a lot of ancient writings of people complaining about how people pronounce things wrong and explain how to pronounce things phonetically. Also poetry is huge. Based on the meter and rhyme, you can figure out how a lot of things should be said.