Mens rea and actus reus: agreement and genitives

Bare ablatives

Latin can put a noun in the ablative and let the case do a preposition's job: at, by, with.

Mutatis mutandis is the cleverest thing in legal Latin: mutatis, having been changed, a perfect participle, plus mutandis, the things that need changing, a gerundive. Once you have changed what needs changing, the rest applies. An ablative absolute in two words.

Ex post facto, from a thing done afterwards, looks like these but keeps its preposition: ex plus the ablative facto. An ex post facto law reaches back to acts done before it was passed.

A noun and an adjective agreeing

The adjective matches its noun's gender. Change the noun and the ending changes:

Nothing about guilt changes between mens rea and actus reus; only the gender of the noun does. More in adjective agreement.

The plurals: obiter dicta, things said in passing, and res judicatae. Judges say dicta constantly, and plenty of lawyers treat it as a singular.

Genitives hanging off a noun

A genitive means "of", and here it follows its noun.

Three of the five are gerunds, the verbal noun English makes with -ing (proving, deciding, standing), declined like a noun: probandi, of proving. See gerunds and gerundives.

Corpus delicti is not a corpse. It is the body of the offence: the proof that a crime happened at all.