Like many languages, English sounds can be represented many different ways - there are some rules, but always some words that don't fit the rules. Probably the vowel sounds that have different spellings used to be pronounced differently to each other, but now sound the same. All you can do is learn the general rules, but also learn the pronunciation of each word individually.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.