Jack Waterford, Editor-at-Large of The Canberra Times, writes: I’d be pretty sure (though I do not have my definitive Lewis and Short Latin dictionary immediately to hand) that apparatus is a fourth declension Latin noun, with a plural apparatus (if you can do the thing with a horizontal line above the US bit) not apparati. Best not try to impress us with (non) classical learning.

In English, the number of words with Latin origin that call for a Latin plural as opposed to an ordinary English plural is very small, perhaps down to half a dozen, and even with Greek, French and Italian added, probably still short of a dozen:  For example, lamina, laminae;  radius, radii; fungus, fungi; addendum, addenda; phenomenon, phenomena; beau, beaux; bacterium, bacteria; cherub, cherubim; prima donna, prima donnas.

Relatively recently invented words formed (usually from Latin or Greek) are likely to have ordinary plurals rather than the plural that might have applied in the original language: thus referendum, referendums than referenda;  forum, forums, not fora, stadiums not stadia etc