These "I" and "E" vowels are formed by putting your tongue forward in the mouth. That's why they're called "front" vowels. With these words, your tongue depresses and folds back a bit. So "O", "A" and ...

This can make learning to spell the words quite tricky! Watch this short video to see some more examples of silent letter words. Vowel sounds are made by the letters 'a', 'e', 'i', 'o' and 'u'.