This is a stem changing verb because in the first person singular, second person singular, and third person conjugations the letter "i" is added after the "f" as follows:
prefiero
prefieres
preferimos
preferis
prefieron
yo prefiero
Tu prefieres
Él prefiere
Nosotros preferimos
Vosotros preferis(there should be an accent on the i)
Ellos prefieren
The second -e becomes -ie when conjugated in the present tense except for when it is conjugated in nosotros and vosotros.
