Diphthongs: vowel sounds that start as one vowel but then pass into some other one, like German "au", "ei", "eu". In English, the sound of "o" in "home" is a diphthong. Monophthongs: vowel sounds that don't change, such as German "a", "ah", "e", "eh" and so on

Diphthong is a vowel sound in which the tongue changes to produce two vowel sound Monophthong is simple vowel. Like in dutch kort of lang vowel