Plural forms of words borrowed from Latin
The Latin plural for neuter nouns ends -a (in the nominative case which is the case we use when adopting Latin nouns into English). The singular ends with -um, in many examples, but not all (caput - capita as in per capita which should really be per caput as it means ‘per’ head, not heads). In English we follow this rule with words we realise are borrowed from Latin, so we have errata for plural erratum, data for plural datum (given ‘thing’, but no one seems to notice that even data is used in the English singular), crematoria for plural of crematorium, corrigenda for things which need correcting, gerundive of obligation of corrigere = to correct. One error needing correcting: corrigendum. These, when they were born, were of course Latin words.
Sometimes not -a, however, for no particular reason. This was mentioned in a recent Daily Telegraph letter to the editor, by a James Wraight of Kent, mentioning mausolea (or mausoleums?). Apparently students at the Royal Military College of Science told their tutor “we have finished the experiment with pendula, have done the sa and are sitting on our ba sucking winega”.
Why pendula? Not pendulums? Pendulum is neuter Latin. Just usage? The other plurals here are of course facetious, (as they are not from Latin), but make the point that the students thought pendula was a bit over the top. Like the story of the charabanc parking spot by Magdalene College at Oxford, signposted “charsabanc”, because technically it was the chars not the banc which were plural (although there were more than one row of bancs in each vehicle the term banc here was used adjectivally, describing how the chars were arranged - in a row, or rows. But the chars in each vehicle were plural too, so perhaps each vehicle should have been called a charsabanc, leaving the pedants nonplussed when it came to pluralising it, as the good bursar’s department of the college must have been doing. So they renamed the vehicle an omnibus, Latin meaning “for all” (ablative masculine/feminine neuter plural) soon abbreviated to ‘bus, as it is spelled in books published up to the Second World War, now just bus, plural buses, not busses because that means kisses.
English isn’t hard, is it?