Some English nouns of foreign origin retain the foreign plural; most of them also have a regular English -s or -es plural, which is often preferred by many writers. We will not be mentioning those regular plural suffixes in our guide, only the foreign inflections.
We begin with the most commonly Anglicized words in the English language, those coming from Latin and Greek.
Rules for foreign plurals of Latin-derived words
For words ending in -a, -a is changed to -ae: antenna to antennae.
For words ending in -um, -um is changed to -a: addendum to addenda.
For words ending in -us, -us is changed to -i: fungus to fungi.
For some words ending in -x, -x is changed to -g and -es is added: phalanx to phalanges.
Rules for foreign plurals of Greek-derived words
For words ending in -ion, -ion is changed to -ia: criterion to criteria.
For words ending in -ma, -ta is added: dogma to dogmata.
Rules for foreign plurals of both Latin- and Greek-derived words
For words ending in -on, -on is changed to -a: phenomenon to phenomena.
For words ending in -is, -is is changed to -es: basis to bases (Latin); hypothesis to hypotheses (Greek). -Ides is also used to pluralize certain -is words: chrysalis to chrysalides (Latin and Greek).