fas

Fas is a neuter noun and completely indeclinable: one form, nominative and accusative, no genitive, no plural. It is related to for, fari, "speak", and means what has been spoken by the gods, so divine law rather than the law of the courts.

That is the pairing to learn. Fas is what heaven permits; ius is what the state permits. Cicero and Livy set them side by side deliberately, ius fasque, to cover both jurisdictions at once, and the opposite nefas means an offence against the gods, an abomination, not a mere illegality. Grammatically it appears almost exclusively as the predicate of est with an infinitive: fas est, "it is right to". The related adjective fastus survives in dies fasti, the days on which courts could lawfully sit; the fasti were the published lists of those lawful court days.