One site is called called the Sounds of English, and it has some helpful information, especially about teaching vowel sounds.
According to the site, there are "two crude rules for decoding. They don't work for all words, of course, but often enough to be helpful for beginners..."
Here are the two rules and some examples:
The Two vowel rule: If there are 2 vowel letters in a short word (syllable), the first vowel sounds like its "alphabet name". These rules work for monosyllabic words or stressed syllables of multisyllabic words.
The One vowel rule: If there is only 1 vowel letter in a short word (syllable), it sounds like a "relative" of the alphabet vowel.
Those rules are for beginning learners, but they were helpful even with intermediate students.