iuro

Iuro, iurare, iuravi, iuratum is first conjugation and perfectly regular. You swear per with the accusative, per deos, and you swear loyalty to something with in and the accusative: iurare in leges, and Horace iurare in verba magistri, "swear by the master words", his phrase for taking somebody else opinion on trust.

The grammatical oddity is iuratus. It is a perfect participle of an active verb but it is used actively, "having sworn", not "having been sworn" - one of a small group that behave this way alongside cenatus and potus. The noun family is ius, iuris, "law", and iusiurandum, "oath", which declines both halves at once: iurisiurandi, iureiurando. Perjury is periurium, swearing straight through the truth.