The difference between the passive voice and the active voice (in any language, not just Talossan) is whether the subject of the sentence is the agent (“doer”) of the verb action, or the object (“doee”) of the verb action.
For example, “the man ate the egg” is an active voice sentence in which the subject is the man, who is the agent (the “eater”), and the egg is the object (the “eaten”). By contrast “the egg was eaten by the man” is a passive voice sentence, one that relates the same facts, but in this case, the subject is the egg (the “eaten”) and the object is the man (the “eater”).
In Talossan, the passive voice is formed exactly as it is formed in English, using a form of the verb estarh (= to be) followed by the past participle of another verb. For example, l’uol esteva menxhat par el cióvec (= the egg was eaten by the man).
Notice also that Talossan puts the object of a passive voice expression (the “doer” of the verb) into a prepositional phrase in the same way English does. Notice the similarity of par el cióvec and the English “by the man”.
Note that the admonition against inflecting the past participle for number and gender only applies to its use in the perfect aspect. In passive voice constructions, the past participle does inflect for gender and number. For example, notice that in la verdù füt menxhada par el cióvec (= the shark was eaten by the man) and las leituras ischent finischadas (= the lectures will be finished), the feminine (menxhada) and feminine plural form (finischadas) of the past participle were used.
The passive voice can of course be seen with expressions in any aspect and tense. For example, a veneva d’estarh veidrestada par l’ensegnistà (= she had just been corrected by the teacher) is a passive voice phrase incorporated with the retrospective aspect in the past tense.