Plurals of Latin/Greek words

     [This is a fast-access FAQ excerpt.]
Not all Latin words ending in "-us" had plurals in "-i".
"Apparatus", "cantus", "coitus", "hiatus", "impetus", "Jesus",
"lapsus linguae", "nexus", "plexus", "prospectus", "sinus", and
"status" were 4th declension in Latin, and had plurals in "-us" with
"genus", and "opus" were 3rd declension, with plurals "corpora",
"genera", and "opera".  "Virus" is not attested in the plural in
Latin, and is of a rare form (2nd declension neuter in -us) that
makes it debatable what the Latin plural would have been; the only
plural in English is "viruses".  "Omnibus" and "rebus" were not
nominative nouns in Latin.  "Ignoramus" was not a noun in Latin.
   Not all classical words ending in "-a" had plurals in "-ae".
"Anathema", "aroma", "bema", "carcinoma", "charisma", "diploma",
"dogma", "drama", "edema", "enema", "enigma", "lemma", "lymphoma",
"magma", "melisma", "miasma", "sarcoma", "schema", "soma", "stigma",
"stoma", and "trauma" are from Greek, where they had plurals in
"-ata".  "Quota" was not a noun in Latin.  (It comes from the
Latin expression quota pars, where quota is the feminine
form of an interrogative pronoun meaning "what number".  In *that*
use, it did have plural quotae, but in English the only plural
is "quotas".)
   Not all classical-sounding words ending in "-um" have plurals in
"-a".  "Factotum", "nostrum", "quorum", and "variorum" were not
nouns in Latin.  (Totus = "everything" and noster = "our" were
conjugated like nouns in Latin; but "factotum" comes from fac
totum = "do everything", and "nostrum" comes from nostrum
remedium = "our remedy".)  "Conundrum", "panjandrum", "tantrum",
and "vellum" are not Latin words.
   If in doubt, consult a dictionary (or use the English plural in
"-s" or "-es").  One plural that you *will* find in U.S.
dictionaries, "octopi", raises the ire of purists (the Greek plural
is "octopodes").
   The classical-style plurals of "penis" and "clitoris" are "penes"
/'pi:ni:z/ and "clitorides" /klI'tOrIdi:z/.
   The Latin plural of "curriculum vitae" is "curricula vitae".
Some people who know a little Latin think it should be "curricula
vitarum" (since vitae means "of a life" and vitarum means "of
lives"); but to an ancient Roman, "curricula vitarum" would suggest
that each document described more than one life.  This is a feature
of the Latin genitive of content, which differs in this regard from
the more common Latin genitive of possession.