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Is there anyway to understand this? Are there any rules concerning pronunciation of English vowels?

To be
Bee
Happy

By
Good bye
Lie
Lice
Mai (how would you pronounce it?)
Neither

Beat
Feet
Pete

Here
Hear
Beer

Bear
Hair
Rare

Ray
Raise
Race

Rat
Red

Food
Do

Poor
More
Boar
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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.

'Mai' isn't an English word. You find it in other words, like 'maize', where it's pronounced like the vowel in 'raise' and 'race'.
Comments  
Unfortunately, pronunciation of the vowels in English is usually based on what language the word first came from. English developed by taking words from so many languages, the variety was too great to produce clear patterns or rules.