There are some languages that have very clear, consistent rules about how words are spelled, and how that spelling translates into pronunciation. Unfortunately, English is not one of those languages.
English started as a dialect of GermanGermanic language, but was then heavily influenced by French (and thus Latin), apparently due to the Norman Conquests. Since then, a variety of other languages have had varying degrees of influence on English. Thus, English is a mixture of many different standards, making it inconsistent.