This document discusses stem changers in Spanish verbs. It explains that some Spanish verbs have vowel changes in the stem when conjugated, and identifies four types of stem changers: E to IE, O to UE, E to I, and U to UE. For each type of stem changer, it provides examples of verbs that follow that pattern and shows how the stem changes in different conjugations, except for the "nosotros" form which remains normal.