Monophthongs v Diphthongs Overview
Vowel sounds In English, there are two types of vowel sound: Monophthongs Diphthongs
Monophthongs e.g. fat thin hot Monophthongs are short, non-gliding vowel sounds. e.g. fat thin hot Notice that the vowel sound in each of these words is short, and does not change.
Diphthongs e.g. crazy boat now Diphthongs are longer, gliding vowel sounds. e.g. crazy boat now Notice that the vowel sound in each of these words is longer, and the stress slightly changes (it glides down usually).
In English soei (beautiful) In English, 99% of all vowel sounds are monopthongs (single) or diphthongs (double). Some languages have triphthongs (Thai for example). This partly explains the difficulty for Thai people when pronouncing English: soei (beautiful) http://www.fonetiks.org/sou4th.html