>>>>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).
>
>Isn't there an "er" one too, like "magister"?
>And an "or" and "ix", as "director" and "directrix", or are they like special case "doer" words, like the English "vendor", "camper", "driver", etc.

I think those generally fall into 3rd declension. My Latin is about 35 years old, so I can't claim I remember everything correctly :).

>Ah, gotcha. So opus is an irregular NEUTER, masquerading as a masculine.

"-us" is not a sure sign of a masculine gender in nouns. All those -us nouns of the fourth declension are neutral. And it's not masquerading as masculine, it is neutral and proudly and regularly so. It's just regular by a rule you weren't aware of :).

Other than that, you got it right :).

>OK, the rest follows without the explanation. Thanks. Still think it should be "opus magnum" though :-)

It is.