If the Formal Affirmative Plural command (Uds) for a stem changing verb is formed by taking the base stem from the yo form and changing the ending to the opposite ending (AR verbs end in E, etc.) Then to make a command out of the verb COMENZAR isn't it "¡comienzen!" ? The answer in my book says it's comiencen.
Using VOLVER the same way, I'm confused why COMENZAR isn't comienzen.
Yo vuelvo = ¡vuelvan!
Yo comienzo = ¡comienzen!
Can anyone explain this with a very to the point and easy to understand explanation? Thanks!