Were the correct translation of operandi “operation”, then the word would be functioning as a noun in the genitive case (the case of possession).
The mistranslation comes from the use of the word operandi. “Modus” is a Latin word meaning “method” or “mode.” It is used to convey the means by which something is done. Modus Operandi is often mistranslated as “method of operation.” However, a closer look at Latin grammar reveals a slightly different grammatical construction. In fact, the word processor used to create this article did not balk at either the spelling or grammatical use of the phrase. Modus operandi (or MO for short) is a Latin phrase so common to English speakers that it appears in its unaltered form in most dictionaries. Discussed below are three common Latin phrases used in English with a brief analysis of the grammar at play. However, as is the case with many Latin phrases, the literal meanings of the phrases have given way to more common language that often betrays its true roots in Latin grammar. Today, English speakers pay homage to Latin by keeping it alive in numerous phrases used in everyday language. During the medieval and renaissance periods of Europe, Latin stood for many centuries as the language of royalty, diplomacy, science, and theology long after the language was no longer a spoken language of an extant people. As a Germanic language, the mother of all Romance languages has oddly influenced English.