Question

what takes place in the conjugation that makes Preferir a stem-changing verb

Answer 1

"Preferir" is a stem-changing (or "boot") verb because its stem vowel (the e in prefer-) changes when that vowel is stressed in certain tenses/moods. The change is a diphthongization e → ie in most present-tense forms and the present subjunctive, and an e → i in certain other contexts (preterite 3rd persons, gerund).

Key points and examples

- Present indicative (e → ie, but not in nosotros/vosotros because the stem is unstressed there):
- yo prefiero, tú prefieres, él prefiere, nosotros preferimos, vosotros preferís, ellos prefieren

- Present subjunctive / affirmative imperative (same e → ie pattern):
- que yo prefiera, que tú prefieras, que él prefiera, nosotros prefiramos, vosotros prefiráis, que ellos prefieran

- Preterite (short past): no change for most persons, but e → i in 3rd person singular/plural:
- yo preferí, tú preferiste, él prefirió, ellos prefirieron

- Gerund and some participial forms: prefiriendo (here the e changes to i)

Why it happens (brief phonological explanation)
- The change occurs when the stem vowel is stressed: stressed mid vowel e in Old Spanish underwent diphthongization to ie in many environments (a historical phonological process). When the stem is not stressed (nosotros/vosotros forms, many past forms), the vowel does not diphthongize, so it stays e. In some -ir verbs like preferir, historical developments produced an e → i alternation in certain past/gerund forms instead of e → ie.

In short: "preferir" is stem-changing because its stressed stem vowel changes (usually e → ie) in the present and related forms; the change is driven by stress/phonology and follows the regular patterns for Spanish stem-changing verbs.