*The stem of a verb is the part left over after the -ar, -er, -ir ending is removed.
*Stem changing verbs use the same endings as regular verbs, but the stem is different. These are sometimes called "boot verbs" because when the forms that have a stem-change are circled, the shape resembles a boot.
*The next-to-last vowel in the infinitive is the vowel that has the stem-change.
Here are verbs in each of the categories:
Here is an example of a stem-changing verb conjugated: