To help with the gap you’re struggling with — it’s critical to understand that a) the demotic and hieroglyphic sections were functionally the same text, 2 different ways of writing the same Ancient Egyptian, and b) the translators had access to the basic structure of the language they were looking for which is preserved in Coptic, used especially in the Christian population of Egypt, which is a more modern version of Ancient Egyptian in roughly the same way that Italian is a modern version of Latin.
Step 1 of using the Rosetta Stone was figuring out how demotic worked — which is a simplified writing style from hieroglyphics, with many but not all characters acting like letters in an alphabet, and which influenced the Coptic alphabet so it wasn’t a total unknown. They knew the translation of the text from the Greek and thus could figure out what sorts of sounds they were looking for by knowing what that text should look like in Coptic. This was still a lot of work, but eventually they were able to be confident that they understood how the demotic worked and could sound out the Egyptian text of the top two sections of the stele.
Step 2 was figuring out the rules for how hieroglyphs worked to make the sounds that they knew from the Demotic script that had now been worked out. Hieroglyphs are more complicated and not really easily directly related to known scripts like Coptic, so they needed demotic first to be able to figure out what the hieroglyphs were saying.
Latest Answers