Grammatical Form and Usage
The phrase isn't grammatically correct in Latin, as it consists of the preposition ex, the preposition post, and a noun with the wrong grammatical case to agree with post. Indeed, the Latin for this phrase is actually two words, ex postfacto, literally, out of a postfactum (an after-deed), or more naturally, from a law passed afterward.
Therefore, ex post facto or ex postfacto is natively an adverbial phrase, a usage demonstrated by the sentence "He was convicted ex post facto (i.e., from a law passed after his crime)." The law itself would rightfully be a postfactum law (lex postfacta); nevertheless, despite its redundant or circular nature, the phrase an ex post facto law is used.
In Poland the phrase lex retro non agit ("the law does not operate retroactively") is often used.
Read more about this topic: Ex Post Facto Law
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