I've been studying ancient Greek for 7 years now and I still don't really understand how accents work. I know basic rules like, circumflex can stand only on long syllables and on first and second syllable from the end of the word, while acute can stand on both long and short syllables, and on first, second and third syllable from the end of the word. When I decline a noun, I know by feeling where the accent should stand, but I don't know which one, and in some nouns, for example "βασιλεύς", it seems like there's no rule, so Ι just have to learn the declension and accents by heart. A lot of nouns and verbs in Greek also have some irregular long or short syllables in declension or conjugation in their basis so that makes it even harder.
So if you have any kind of advise, please, it will help a lot!