1. A diphthong consists of two vowels that produce but one sound. The second vowel is always an ι or an υ. They are pronounced as follows. (The diphthong ωυ is used in Classical Greek, but occurs in the New Testament only in the name Μωυσῆς where there is always a diaeresis over the ϋ indicating that it is not a diphthong.) You can download a diphthong chart to help you learn these vowel combinations.
αι as in aisle αἴρω ει as in eight εἰ οι as in oil οἰκία αυ as in sauerkraut αὐτός ου as in soup οὐδέ υι as in suite υἱός ευ, ηυ as in feud εὐθύς or ηὔξανεν. Some suggest that the pronunciation of ηυ is the same as saying "hey you" if you run the words together. υι and ηυ are less common than the others.
2. An improper diphthong is made up of a vowel and an iota subscript. An iota subscript is a small iota written under the vowels α, η, or wω (ᾳ, ῃ, ῳ) and normally is the last letter in a word. This iota has no effect on the pronunciation but is essential for translation, so pay close attention to it.
3. If a word begins with a diphthong, the breathing mark is placed over the second vowel of the diphthong (αἰτέω).
4. In some words we find two vowels that normally form a diphthong, but in this case do not. To show that these two vowels are pronounced as two separate sounds, a diaeresis is placed over the second vowel. A diaeresis is two small dots. The αι normally forms a diphthong, but in this word the diaeresis indicates that it forms two separate sounds: ᾽Η σα ϊ ας. Cf. naïve in English.