There are two passive-voice forms in Gilkesh. One of these is called the passive (as in "patient", the entity undergoing or "suffering" the action of the verb) and the other is the "receptive" voice. (These terms replace the somewhat confusing terms I introduced earlier.)
The passive (patient-subject) takes the nu- prefix.
Sudritid sapire gela. [She gave the book to the girl.] -> Nusudritid sapiru gela. [The book was given to the girl.]
The receptive (goal-subject) form takes the mu- prefix; the effect of mu- is to reverse the arrow of action. In verbs of conveyance, this corresponds to a less common usage of the passive form in English expressing the recipient of the action:
Sudritid keshu sapire gela. [The woman gave the book to the girl.] -> Musudritid gelu sapire kesha. [The girl was given the book by the woman.]
Notice that with "mu-", not only does the goal (final endpoint) of the action become the subject of the verb, but the agent (the doer of the action) becomes the direct object; in other words, "mu-" has the effect of interchanging the roles of the grammatical subject (nominative -u ending) and direct object (objective -a ending).
In many simple passive-voice constructions, mu- and nu- may be used interchangeably:
Munangitid abu. = Nunangitid abu. [The water was drunk.]
Relative clauses are most often formed with the particle di. The di particle can be rendered as "such that" but it doesn't translate directly. Because Gilkesh word order is VSO, the syntax of relative clauses comes out a little bit different than in a SVO language like English:
Raqiti kesha di-ekishmid dha. [I saw the woman whom I had loved. Literally, "I saw the woman such-that I had loved whom."]
The fourth abstractive.
Here we meet our new friend, the Fourth Abstractive. It's really our old friend, the infinitive, with case endings. The fourth abstractive is basically a gerund:
Avnitid sudriyu Susanu sapire Joana Maria. [Susan's giving the book to Joanne pleased Mary.]
Avnitid Maria sudriyu Susanu sapire Maria. [Susan's giving the book to Joanne pleased Mary.]
The subject/object order may be reversed for clarity, as here.
Using the receptive voice (goal-subject):
Muavnitid Mariu sudriya Susannu sapire Joana. [Mary was pleased by Susan's giving the book to Joanne.]