Take, for example, 'ignoramuses' instead of 'ignoramae', or 'cacti' over 'cactuses'?

In which cases does the plural end in 'es' instead of 'ae'? Can it be either one for any given case? Why?

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    This is a knotty subject. This sort of thing happens in words taken from Latin or Greek: in an earlier age, when anybody who was well enough educated to know these words also knew what the Latin or Greek plural was. Today, we're left with a few preserved forms. Some people like to show off their learning by using Latin plurals in English: sometimes they don't know enough Latin to know what the correct Latin plural is (for example, the words status, prospectus, octopus, and corpus do not form their plurals in Latin by changing -us to -i, but some people use such forms in English. ... – Colin Fine Apr 17 '16 at 21:50
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    ...(continued) My advice is to use a Latin or Greek plural only if you're sure you've seen it used, and otherwise just treat it as an English word. Except for some quite common examples like "cacti", most people won't be bothered. – Colin Fine Apr 17 '16 at 21:51
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    @ColinFine The OED does list octopi as one of three possible plurals of octopus. Plural octopuses, octopi, (rare) octopodes Brit. Also prospecti. – WS2 Apr 17 '16 at 22:17
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    @ColinFine: The problem is that "octopus" is not even a word in Classical Latin, so there is no precedent there. It was invented as part of modern Scientific Latin (apparently by Linnaeus). There's a good overview here: english.stackexchange.com/a/138236/77227 – herisson Apr 17 '16 at 22:21
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    The plural of ignoramus is ignoramuses. The singular is not a Latin noun form, so don't inflect it for number. – deadrat Apr 18 '16 at 0:25

It is the plural of Latin origin used mainly in formal contexts. In ordinary speech the -es suffix is commonly used:

Catci vs cactuses:

  • Cacti is the Latin plural of cactus, and some writers use it in English. Cactuses is the English plural. Dictionaries list both, and neither is right or wrong. Also, like many names of plants, the uninflected cactus is sometimes treated as plural.

  • The prevalence of the Latin cacti can be attributed to the influence of Latin on biological nomenclature.

  • These Latin plurals are not considered out of place in botany and other scientific fields, and some make their way into broader usage, but there’s no good reason that the ordinary English speaker should have to abide by the rules of Latin grammar.

  • Cactus is not the only Latin-derived English word ending in –us, and most are conventionally pluralized in the English manner. Fungus, like cactus, often becomes fungi (though funguses is just as good), but this is one of the few exceptions.

  • Most English speakers don’t say ani instead of anuses, apparati instead of apparatuses, campi instead of campuses, octopi instead of octopuses, stati instead of statuses, or viri instead viruses, and there’s no reason cactus should be any didfferent. It’s a matter of preference, though, and cacti is not wrong.

(The Grammarist)

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    And stati and viri are not good Latin anyway. The Latin plural of status is status, and virus has no recorded plural (it was generally uncountable in Latin) but since it was neuter, its plural must end in -a. And as for octopi! – Colin Fine Apr 17 '16 at 21:55
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    Mr Grammarist obviously does not know any Latin and should not be trusted about English either. – fdb Apr 17 '16 at 23:40
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    I would omit the last bullet point, since it mars your post's credibility. In addition to what Colin Fine has said, the plural of apparatus is apparatus both in Latin and in English (I have never encountered apparatuses, although my spell-checker approves it), and the only correct Classical plural of octopus is octopodes, not octopi. – Anonym Apr 18 '16 at 1:45
  • @fdb - I don't think the source is unreliable and the Latin exemples are made on a colloquial basis, that is , not knowing the exact plural form in Latin people would probably guess at it. – user66974 Apr 18 '16 at 7:32

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