I should have known
At the end of the book, Beers includes in the Appendices all of the spelling and word building rules that I should have picked up along the way. I didn't know that there was a rule about when to add -ible instead of -able. I learned "when two vowels go walking...." at some point, but I didn't "know" the rule enough to demonstrate it until I saw another K teacher doing it.
For so many things in the English language, I've heard teachers say "It just sounds right." Well, guess what - to our ELLs, it doesn't just "sound right."
I plan to study and learn (enough to be able to explain and teach) the advanced phonemic principles. Many of my students need these tools.