English speakers, especially those from the southern US, tend to "stretch" or distort vowels giving rise to what are called diphthongs. A diphthong is actually a combination of two sounds, a vowel and a glide or semi-vowel. Pronounce the English words I, out, and boy and you will note that the vowel actually changes its or sound. Try saying each word slowly and note how your jaw moves during the vowel.
Unlike English, the vowels in French are never followed by a glide. You must learn to keep your lips, tongue and jaw stationary during the pronunciation of a French vowel, which will result in a pure sound. If you relax your mouth or jaw, you will produce diphthongized vowels that will give your French an American accent.
Like all French vowels, the vowels /i/ /y/ /u/ require much greater muscular tension than do English vowels. Listen to the following words and note the difference in vowel quality (tense vs. lax) between French and English:
Activité 1. Lisez les mots suivants.
Aknowledgment: some parts of this page are partially adopted from Francais Interactif.