Vowels are long when they are in open syllables, which end in a vowel.
They are short when the are in closed syllables, which end in a consonant. Aa, ee, ie, oo, and uu are used to represent long vowels in closed syllables. Doubled consonants are pronouced the same as single ones, and are used to make the preceding vowel short. The apostrophe in enclyctics such as 't are schwas. Click here for some grammar.