Combinations of vowels
ae ea ai ia ao oa au ua ay ya
ei ie eo oe eu ue ey ye
io oi iu ui iy yi
ou uo oy yo
My suspicion is that if any of these combinations occurs in a syllable of a word, one letter has a fair chance of becoming silent. I may look into it soon to see if I can find a general set of rules.
Stay tuned