Question

which two verb forms do not undergo a stem-change
ella & ellas
nosotros & vosotros

usted & ustedes
yo & tu

Answer 1

The two verb forms that do not undergo a stem-change in the present tense in Spanish are "nosotros" and "vosotros." In most conjugations, these forms maintain the original stem of the verb, unlike the other provided pairs which can exhibit stem changes.