Avancemos 1. U4L1. Verbos con cambios en la raíz e-ie

Stem-changing verbs change more than just the endings when you conjugate them. The vowel in the stem (the part of the verb left when you remove the -ar, -er and -ir endings; the second-to-last syllable) will change in these particular verbs from a single vowel to a double vowel.
However, these verbs do not stem-change in the nosotros or vosotros forms.