Know how to use "fewer" and "less"? Find out.
occasional plural suffix of words ending in -a, most of which, in English, are from Latin nominative fem. singular nouns, which in Latin form their plurals in -ae. But plurals in -s were established early in English for many of them (e.g. idea, arena) and many have crossed over since. Now it is not possible to insist on purity one way or the other without breeding monsters.