In Proto-Indo-European, You Had Zero Grade, E-Grade, and O-Grade.

In the, in the Proto-Indo-European situation, you had what were called grades, vowel grades. You had zero grade, E-grade, and O-grade - no vowel at all. In Greek, it's somewhat predictable for certain kinds of stems, but as phonological processes take their toll, you get the insanity of English or Germanic languages where you have five, six, seven verb classes with different kinds of vowel changes.