I've written twice now about changing a vowel syllable to an H+vowel syllable when two vowels are next to each other that do not make a legal diphthong, but I realized I've never explicitly said what a legal diphthong is.
There are three: ei, ai and ua (ɛi, ɑi and øɑ). Other combination of vowels take an h between them in spelling if they are created when forming a compound word. Non-compound words don't place other vowels next to each other.
I imagine in quick speech the h is sometimes dropped, and/or replaced with a glottal stop.