Diphthongs , Monophthong - JSS1 English Language Lesson Note

Diphthongs are special sounds in English made by combining two vowel sounds into one smooth sound.

oi" - This makes the sound in "boil," "join," and "voice."
"ow" - Hear it in "cow," "how," and "town."
"ai" - Like in "rain," "paint," and "wait."
"ei" - Listen for it in "rein," "reign," and "weight."
"oy" - This one's a bit tricky! Try "toy," "boy," and "joy."

What is a Monophthong?:

A monophthong is simply a vowel. Derived from the old Greek language (with Mono meaning one/single and the -phthong meaning sound/tone and  the basic word phthalein which means to speak/create sound with the voice), the word monophthong  shows that a vowel is spoken with exactly one tone and one mouth position. Take for example when you say “teeth”; while you are creating the sound of the “ee”, nothing changes for that sound.

A monophthong can be a lexeme of a language and as such it can as well be a syllable. There is no shorter syllable than a single monophthong