So this is where we are: there are 6 vowel letters, ‘a,’ ‘e,’ ‘i,’ ‘o,’ ‘u,’ ‘oo,’ 4 “contexts,” lax, tense, r-lax and r-tense and 18 vowel sounds, 12 of which can be spelled by 2 letters, ‘aa,’ ‘ae,’ ‘ai,’ ‘au,’ ‘ee,’ ‘eu,’ ‘ia,’ ‘oa,’ ‘oi,’ ‘ou,’ ‘ue,’ ‘ui.’ Each vowel letter has a specific way of pronouncing it for each of the contexts, and if that isn’t the way that’s wanted, the fallback is to use the 2-letter spelling.
The lax context is before 2 or more consonant letters, unless the first is an ‘r’ and the second is not, or before a single consonant if it’s at the end of the word. Any context can be made lax by doubling the consonant, as in ‘holly’ (not ‘holy’) or ‘marry’ (not ‘Mary’).
The tense context is before a single consonant letter except ‘r’ followed by a vowel, or the very end of the word.
The r-lax context is before ‘r’ and a consonant that’s not ‘r’, or before ‘r’ at the end of a word.
The r-tense context is before ‘r’ and a vowel.
So a final vowel is pronounced tense, as in ‘caffa,’ ‘fli,’ ‘hello,’ ‘cu,’ ‘moo,’ with an exception for ‘e’: a final ‘e’ is not pronounced, but still provides the context to make a preceding vowel tense or r-tense. If the word is one syllable (only consonants before the ‘e’), the e is not magic—there is nothing for it to magic—so it’s still pronounced normally, as in ‘he,’ ‘she,’ ‘fle,’ ‘tre.’ So how to get a final ‘e’ sound in a multisyllabic word? Magic ‘y,’ that’s how, as in ‘coffy,’ ‘silly’ or ‘acmy.’ Final ‘y’ is magic because it’s not a consonant, it’s a vowel. Luckily, a word can’t end in a consonant ‘y’—that’s just the case, it seems—so we don’t need any more rules after this.
Late update: we do need more magic. Some authorities allow for a word to end with a short vowel, and we have no way to represent that. So we need magic ‘h’, a final consonant which still shortens the previous vowel, but is not sounded itself. This is used if ‘dhe’ is pronounced ‘dhuh.’