A few of my students are also high school teachers and have asked the same question) have asked me why there is not just one standardized spelling rule for "y"-ending pluralizations. Example: currently, the plural (and/or conjugated form) of "fly" would be "flies" but the plural of "play" would be "plays" (the difference is due to the vowel/consonant ending in the root). Why not "flys", for example, to coincide with "plays"? In other words, why not simply standardize the form so that anyone could simply add an "s" to any word ending in a "y" when pluralizing?
THANK YOU FOR YOUR TIME!!