What are stem-changing verbs in Spanish and how do they affect verb conjugation?

Stem-changing verbs in Spanish are verbs that have a change in the vowel of their stem when conjugated. This change only occurs in certain forms of the verb, such as in the present tense. The most common stem changes involve e to ie, e to i, o to ue, and u to ue. These changes affect the pronunciation and spelling of the verb, but the overall conjugation pattern remains the same.