Stem Changing Verbs

I learned that the stem of a verb is what is left once you take away the -ar, -er, or -ir.  I learned that there are three common types of stem changing verbs.  Those types consist of changing "e" = "ie", "e" = "i", or "o"="ue".  Examples in Yo form would be Comenzar = Comienzo, Medir = Mido, and Dormir = Duermo