I’d like to know why ‘naive’ is pronounced ny-eve but is spelt naive. Where is the ny part coming from? ‘na-‘ isn’t pronounced ny, and if the ny part is nai-, then there is only -ve left. This is about pronunciation
French naïf/naïve does not have a falling diphthong, so its pronunciation wouldn’t explain the use of /aɪ/ in English.
English doesn’t have that many words where /ɑ/ comes directly before another vowel, especially not when the /ɑ/ is in a “weak” position (unstressed, or at least directly before another vowel with a higher level of stress). Words with /aɪ/ before a stressed vowel are a bit more common; e.g. biography, diameter.
The replacement of /ɑ/ with /aɪ/ is presumably an assimilation to the /i/ in the next syllable. In other contexts, other vowels may turn into diphthongs before a palatal glide /j/ in the following syllable: for example, lawyer has come to be pronounced with the /ɔɪ/ diphthong in the first syllable for many people, and words like arroyo, Loyola, Toyota likewise have /ɔɪ/ instead of /o/ as a syllable nucleus.
Something else that might be relevant is that apparently, people once used the pronunciation “/neɪˈiːv/”, with an anglicized “long a” value for the first vowel. This might have made it easier for the diphthong /aɪ/ to eventually become established. Other words spelled with “a” that had variants with /eɪ/ and that are now often pronounced with /aɪ/ (rather than ɑ) in its place are Naomi and Israel. Dais also can have either /eɪ/ or /aɪ/ in the first syllable (the French source word was a monosyllable, but the English pronunciation has deviated in syllable count).