Why is great pronounced /greit/ while in other words the ea is pronounced differently? Take treat, for example: /tri:t/. Why are two words with the same number of vowels and consonants and the same syllable structure are pronounced differently? Is there any way for people to guess a word's pronunciation at first place correctly? How can we trace pronunciation history of a word?
Perhaps this will help sort out why English vowel structure appears to have inconsistent pronunciations (and its origins):