Conjugate the verb preferir in all forms in the Present Tense.

What takes place in the conjugation that makes this verb a stem-changing verb?

The verb preferir conjugated in the Present Tense is as follows:

Yo prefiero (I prefer)
Tú prefieres (You prefer)
Él/Ella/Usted prefiere (He/She/You prefer)
Nosotros/Nosotras preferimos (We prefer)
Vosotros/Vosotras preferís (You all prefer)
Ellos/Ellas/Ustedes prefieren (They/You all prefer)

The stem-changing that takes place in the verb preferir is the e in the stem changes to ie in all forms except for the nosotros and vosotros forms.