explain how stem changing -ir verbs are different from stem changing -ar and -er verbs in the present subjunctive
Stem changing -ir verbs are different from stem changing -ar and -er verbs in the present subjunctive because they have a different pattern of stem changes.
For stem changing -ar and -er verbs, the stem changes in the present subjunctive only occur in the nosotros/nosotras and vosotros/vosotras forms, where the e in -er verbs changes to an a, and the o in -ar verbs changes to a ue. For example, the verb pensar (to think) would change to piense, pienses, piense, pensemos, penséis, piensen in the present subjunctive.
On the other hand, stem changing -ir verbs have stem changes in all forms except the nosotros/nosotras form. The e in the stem changes to ie, and the o in the stem changes to ue. For example, the verb servir (to serve) would change to sirva, sirvas, sirva, sirvamos, sirváis, sirvan in the present subjunctive.
Thus, the key difference between stem changing -ir verbs and stem changing -ar and -er verbs in the present subjunctive lies in the pattern of stem changes for each verb group.