-as


-as
-ae, -as
as plurals of nouns in -a. Most English nouns in -a are from Latin (or Latinized Greek) feminine singular nouns, which have in Latin the plural ending -ae. But some have a different Latin origin: e.g. subpoena is not nominative, comma and drama are neuter forms, and addenda, data, and stamina are plurals, and so with these words plural in -ae is not possible. Other words are not from Latin at all: e.g. sofa is from Arabic and swastika is from Sanskrit.
Of those words that are genuinely able to have plurals in -ae, some more technical ones do so (algae, larvae), whereas those in general use form English plurals in -as (areas, ideas, villas) and those in both technical and general use have both forms depending on the domain of use (antennae or antennas, formulae or formulas, nebulae or nebulas).

Modern English usage. 2014.