1. Monophthongs v Diphthongs
Overview

2. Vowel sounds In English, there are two types of vowel sound:
Monophthongs
Diphthongs

3. 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.

4. 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).

5. 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)