I learned that the word "dyadic", a word opposed to monadic, is written with a y.
My etymology sources indicates it comes from "Dyad", which means "pair", "couple", "double". However, every other word I know with a "di-" prefix meaning "two" is written with an i. (Dioxyde, dipterous, dioptre, digram, ... The "bi-" prefix is now much more prevalent but every other case of "di-" I know uses an i)
Even in ancient Greek, there's a difference between the "δι-" prefix and the word "δυάς", they don't use the same letter, although they both come from the Greek word for "two".
Why is there a difference in spelling between the Greek word for two and the "di-" prefix?
Why does "dyadic" seem to be the only word that kept the y?
Note that this seems to date from ancient Greek, so as far as I know it also happens in every language that got these words from Greek, not just English.
Note2: I should have specified it, but of course I'm talking about words where the "dy-" prefix means "two", so this is not about words like "dynamic" or "dyssentry".