The past participle of ser is sido. The gerund form of ser is siendo. Let’s see them in action. Yo he sido una maestra por mucho años. – I have been a teacher for many years. Hemos sido amigos por solo un par de semanas. – We have been friends for only a few weeks.

To form these, we must conjugate the auxiliary verb haber and add the ser past participle, which is sido. For explanations of these two components of compound verbs, check out our post on the Spanish past participle, and the auxiliary verb haber.