Italian vowels (i vocali) are short, always pronounced very clearly, and are never drawn out. The "glide" with which English vowels frequently end should be avoided. It should be noted that a, i, u, are always pronounced the same way; e and o, on the other hand, have an open and a closed sound that may vary from one part of Italy to the other.
Also, when vowels are grouped together in a word, they are always pronounced separately.
The approximate English equivalents are as follows:
In italian a diphthong is formed by any vowel preceded or followed by an unstressed closed vowel ('i' or 'u'):ia, ie, io, ai, ei, oi, ua, ue, uo, au, eu, ou, iu, ui. They are always pronounced maintaining the sounds of the individual vowels, and the closed vowel plays the role of a semivowel or a glide.