n Thai there are fifteen dipthongs which can be classified into two groups, depending on the role the length of the vowels play in the word meaning.
1. Short and long vowel sounds coming as pairs, with difference in meaning when pronounced.
[e:U] [eU] [a:I] [aI] [a:U] [aU]
as in as in as in
เละ / เล อาย / ไอ ดาว / เดา
2. Those vowel sounds, without difference in meaning no matter how long they are pronounced.
[E:U] as in แมว [o:I] as in โบย [I: ́] as in เปย
[F:I] as in เลย [μ: ́] as in เรือ [uI] as in คุย
[I:U] as in หิว [ç:I] as in คอย [U: ́] as in วัว
Thai students hardly have problems with English dipthongs, except [ ́U] as it does not appear in Thai vowel sounds. And they tend to substitute it with [o] when pronouncing any word with this vowel.