Part of my quest to reinstate Latin as the lingua franca of the modern world involves bringing Latin plurals back into common usage. Here are five simple rules to ensure that you are never embarassed in polite company.
1. Words ending in -a change to –ae
one formula, two formulae
one antenna, two antennae
one banana, two bananae
2. Words ending in –us change to –i (see exceptions below)
one focus, two foci
one locus, two loci
one walrus, two walri
3. Words ending in –um change to –a
one datum, two data
one referendum, two referenda
one alpacum, two alpaca
4. Words ending in –x change to –ces
one appendix, two appendices
one index, two indices
one box, two boces
5. Words ending in –is change to –es
one crisis, two crises
one thesis, two theses
one iris, two ires
People often try to apply rule #2 to the words platypus, octopus and hippopotamus. These words are obviously of Greek origin, and so applying Latin plurales to them is problematic. The correct form for the first two is of course platypodes and octopodes (pus/pod- means 'foot' and is the same word that you find in podiatrist). Hippopotamus is a bit trickier. As you should know hippo is Greek for ‘horse’ and potamus means ‘of the river’. Therefore the correct plural should be hippoipotamus ('horses of the river') or if you like hippoipotamon ('horses of the rivers').
Neuter nouns such as opus and genus and gerbus, though being Latin in origin, also do not conform to rule #2. Their plurals should be opera, genera and gerbera respectively.