akto

An act you perform is not an akto. This is the trap, and it costs beginners a sentence a day. Agi is to act, and the noun built from it is ago, a deed or an action: bona ago, a good deed, ago de kuraĝo, an act of courage. Akto is a separate borrowed root with two narrow senses, and neither is "something somebody did". Where English says an act of kindness, Esperanto says ago. Where English says Act Three, Esperanto says akto. The two words are cousins in Latin and strangers in Esperanto.

Two senses, both real, both narrow. The first is theatrical: the divisions of a play, kvinakta dramo, a five-act drama, with sceno for the scene inside the act. The second is legal and administrative: a formal written instrument, a deed, a certificate, a record. Naskiĝakto is a birth certificate, geedziĝakto a marriage certificate, notaria akto a notarial deed, and the plural aktoj is the proceedings of a congress, published. Both senses are about a thing set down in fixed form, which is the thread back to Latin actum.

And aktoro is not akto plus a suffix. Esperanto has no agent suffix -or-; people are made with -ist- (instruisto) or with the participle (aktanto would be one who acts). Aktoro is its own borrowed root, imported whole from Latin actor. So the chapter offers you three words that look like a family and are three separate imports: agi, akto, aktoro. Only the first one builds.