How could praetors use legal formulas to change substantive rules of Roman law?

Praetors in ancient Rome utilized the formulary system to adapt and evolve Roman law by issuing edicts that outlined the procedures they would follow during their tenure.1

Under this system, the praetor acted as a judicial officer who oversaw the preliminary stage of a trial. Instead of relying on the rigid, oral legis actiones—where a single verbal error could cause a plaintiff to lose their case—the praetor provided written instructions to the judex (judge). By approving or disapproving these written formulas, the praetor exercised significant control over the legal process.23

The praetor could effectively modify substantive law by:
* Refusing or granting actions: By withholding the order to proceed to the judex, the magistrate could prevent the enforcement of outdated or inequitable claims.3
* Incorporating equity: The praetor used their edict to introduce principles of equity and good faith, allowing the law to adapt to new social and economic conditions.1
* Directing the judex: The written instructions defined the legal parameters of the dispute, enabling the praetor to shape how the law was applied to specific cases.3