A. Only gerunds for –ir verbs undergo a stem change. ... The verb sentir becomes sintiendo. For o-ue stem-changing verbs, change the o to a u. The verb dormir becomes durmiendo.
B. To form the present progressive, we conjugate the verb estar in the present and we add the present participle of dormir, which is durmiendo. Notice the change in the stem vowel from o to u!
