Foreign Words

An interesting lesson from Illustrated Lessons in Our Language by G.P Quackenbos, 1882, discusses how to create plurals from foreign words:

Some English words of foreign origin retain their foreign plurals, changing us to i, is to es, and um or on to a: as,

Alumnus, alumni.
Crisis, crises.
Animalculum, animalcula.
Phenomenon, phnomena.

Some words of foreign origin take both the foreign and a regular English plural; as,

Beau, beaux or beaus.
Cherub, cherubim or cherubs.

English is full of foreign words. So many, in fact, that a great deal of them shouldn't really be considered "foreign" any longer. Quackenbos lists many Latin words which may be considered so thoroughly incorporated into the English language as to be English. After all, many English words have Latin or Greek roots. The exercises following the above lesson present several challenges. Decide if the following words are plural or singular, then see if you know what its opposite is:

parentheses
gymnasium
aquarium
effluvium

(Let me know if you want me to post the answers in the comments :)