Stem-changing verbs, also known as boot verbs, undergo a vowel change in the stem of the verb when conjugated in the present tense. For e → ie stem-changing verbs, the “e” in the stem changes to “ie” in all forms except for the first and second person plural (nosotros/nosotras and vosotros/vosotras). This pattern is common and essential to master for fluency in Spanish.
Now, let’s take a look at some examples (below). Look for how the “e” in the stem changes to “ie” in all forms except nosotros and vosotros. Remember, the forms that change are underneath the “boot” (see examples below):
(Notice how the “e” in Querer changes to “ie” under the “boot”)
(Notice how the “e” in Entender changes to “ie” under the “boot”)
(Notice how the “e” in Pensar changes to “ie” under the “boot”)