If two vowels appear in the middle of a word, and one of the vowels is the letter’I’ , is the ‘I’ always pronounced as a ‘Y’ ?
If “i” is the first letter of the entire word (not counting prefixes, so “adiuvare” is an example), then yes. Otherwise, only in the combination “ei” and in the words huius, cuius, huic, cui, and hui. (“Fuit”, for example, is “foo-it”.)
“Ai” is not a diphthong because the corresponding diphthong is spelled “ae”. Same with “oi” and “oe”.