Fine and Thin

Fine and Thin, Ape and Apple, Unit or Supper... why do some words have their vowels pronounced as "ai", rather than "i", "ae" rather than "a", "iu" rather than "â" and so on?

Here's a rule: How many consonants are after the vowel? If it's one only, so you should pronounce the vowel   as the long version of it ("ai", "ae", "iu"...), and if it's two, or if the word ends with that consonant, so it's pronounced as the short version ("i", "a", "â"...). Got it?