>>>Still on Latin, you're a scholar, I'd always understood that latin adjectives agreed with their nouns, such as "Bigus Dicus". And Magnus is very typical, being of the 3 main declensions: "us", "a" and "um".
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>>>Why is it then that they refer to artists' "Magnum Opus"? Shouldn't that be "Opus Magnus"?
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>>There are five declensions in Latin that I learned, and opus goes by fourth, which is neutral nouns ending on -l, -en, -t, -ar, -ur, -us (not all nouns with these endings, though - just the neutral ones).
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>The 3 main declensions I learnt were like "lupus", "Puella" and "pratum".
>You'd get "Puella magna", "lupus magnus" or "pratum magnum". So, you haven't answered my question: shouldn't "Magnum Opus" (great work?) be "Opus Magnus", also given that the adj. should follow the noun?

It does follow the noun. Magnus is masculine, magna feminine and magnum neutral. Since opus is also neutral, "magnum opus" is correct. Also "magnum crimen" - i.e. it doesn't necessarily mean that the suffix would be the same (because nouns may have different suffixes, and the adjectives have their own, which may or may not be the same), but the gender must be.