French stem-changing verbs are conjugated with the same endings as regular -ER verbs but have two different radicals or stems. Stem-changing verbs are sometimes also called boot verbs or shoe verbs, because if you circle the forms that have stem changes in a certain style of conjugation table, the resulting shape looks like a boot or shoe.
There are six different kinds of stem-changing verbs based on the final four letters of the verb. The actual spelling change required for each type of stem-changing verb is different, e.g., y changes to i in -oyer verbs and é changes to è in -é_er verbs, but the tenses and grammatical persons which undergo the stem change are the same. For example, in the present tense, the je, tu, il, and ils forms of the six different kinds of stem-changing verbs all have a stem change. So once you learn which conjugations need a stem change for one type of stem-changing verb, you will know which conjugations need the stem change for all the other kinds. There's only one exception to this, which you'll learn about in the detailed lessons on stem-changing verbs:
Note: Stem-changing verbs are not the same thing as spelling change verbs.