-iĝ-

-iĝ- is the mirror of -ig-: nobody does it to you, you simply end up in the state the root names. Fari is to make, fariĝi is to become. Sidi is to be sitting, sidiĝi is to sit down. Levi is to lift, leviĝi is to rise. Edzo gives edziĝi, to get married. The result is always an intransitive verb, and it cannot take an object.

Two things catch learners out. Naski is to give birth, so naskiĝi is to be born, and the two are easy to swap under pressure. And -iĝ- is not the passive: la pordo malfermiĝis says the door came open, with no hand implied, while la pordo estis malfermita says somebody opened it. Choose by whether an agent exists.