Cherubim, Seraphim

The words cherub and seraph, are singular.  Cherub, as applied to a little child, takes the English plural, cherubs.  As applied to an order of angels, it takes the Hebrew plural, cherubim.  The singular, seraph, has an English plural, seraphs, as well as the Hebrew plural, seraphs.  The double plurals, cherubims and seraphims, although found in the King James version of the Bible, are regarded as faulty in modern writing, and should be avoided.

News

Although plural in form, the word news is singular in meaning; as,
“The news from Europe this morning is quite interesting.”
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Acoustics

Names of sciences ending in ics, are generally regarded as singular.  “Acoustics is a very considerable branch of physics.”  Do not say, “The acoustics of this hall are good,” but “The acoustic properties of this hall are good.”

Dialectics, dynamics, economics, mathematics, ethics, politics, tactics, when used as substantives, require a verb in the singular.

Analysis

Many words like analysis, crisis, ellipsis, emphasis, hypothesis, oasis, parenthesis, synopsis, form their plurals by changing the termination is into es; as, analyses, crises, etc.  The word iris takes the English plural irises; Latin plural is irides.  Chrysalis has only the Latin plural, chrysalides; but chrysalid, which means the same as chrysalis, takes the English plural, chrysalids.

Terminus

Terminus, radius, alumnus, and some other words ending in us, form their plurals by changing the termination us into i; as termini, radii, etc.

Many words ending in us that formerly were written with only the Latin plural, are now given an English plural also; as, focuses, foci; cactuses, cacti; sarcophaguses, sarcophagi; convolvuluses, convolvuli ; funguses, fungi; nucleuses, nuclei.
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Isthmus, prospectus, rebus, take only the English plural.

Apparatus has no plural.  Avoid apparatuses.

The plural of genius, as applied to a man of unusual vigor of mind, is geniuses.  When applied to a good or bad spirit, the plural is genii.

Formula

Formulas, larvas, stigmas, are regular English plurals; formulae, larvae, and stigmata are the classical plurals.  Nebulae and alumnae are the proper plurals, the latter being the feminine noun corresponding to the masculine plural alumni.

Datum, Phenomenon

Datum, erratum, candelabrum, and memorandum form their plurals by changing um to a; as, data, errata, etc.  The last two also take the English plurals, memorandums, candelabrums.

The plural of phenomenon and criterion are phenomena, criteria, although criterions is sometimes employed.