Stem-changing –ir verbs are different from stem-changing –ar and –er verbs in the present subjunctive because the stem change only occurs in the "boot" or "shoe" of the verb, while the endings remain the same. This means that the conjugation of the verb will have a different stem in certain forms but will still use the same subjunctive endings (-a, -as, -a, -amos, -áis, -an). Stem-changing –ar and –er verbs, on the other hand, have stem changes throughout the conjugation, not just in the "boot" or "shoe" forms.
