Here's why:
* Monophthong: A single, pure vowel sound. Your mouth position doesn't change while producing it. Examples: /ɑ/ (as in "father"), /i/ (as in "see"), /u/ (as in "boot").
* Diphthong: A combination of two vowel sounds that glide smoothly from one to the other. Examples: /aɪ/ (as in "buy"), /ɔɪ/ (as in "boy"), /aʊ/ (as in "how").
Let me know if you'd like more examples!