French is a language with lots of quirks, exceptions, and ways to spell every sound. The long ‘i’ (eye) sound is no exception. Here are some tips and tricks for tackling it.
The long ‘i’ sound doesn’t appear at the beginning of any words derived purely from French. When it appears, it is generally because the word has passed through another language in addition to French, like Greek or Latin. In these cases, it’s best to spell it simply with the letter ‘i’. Some examples are ‘ideal (Greek to Latin to French plus an English combining form) and ‘isocheim’ (Greek to French).
In the middle of a word, pure French rules dictate that you spell it ‘aill’, like in ‘paillard’, or with ‘ai’, like in ‘camaïeu’. When there is influence from another language, you should spell the long ‘i’ sound with an ‘i’ or ‘y’, depending on the context. In words like ‘hydrocephalus’, where there are Greek stems, you should go with a ‘y’, while ‘i’ is preferable in cases like ‘Viorna’, a genus of North American Vines that went from Latin to French to New Latin.
At the end of the word, it can be tricky to pick a way to spell this sound. ‘-aille’ is probably your best bet, showing up in words like ‘rocaille’ and ‘tenaille’. ‘-aie’ shows up in ‘sagaie’ and ‘-ai’ shows up in ‘assegai’, but there aren’t many other notable options for pure French. When there is clear influence from another language, you should spell it as you would in that language.