The Formation of Plurals - Appendices: Some Useful Lists - The Ultimate Introduction to Grammar and the Writing of Good English - Gwynne's Grammar 

Gwynne's Grammar: The Ultimate Introduction to Grammar and the Writing of Good English (2014)

Part III. Appendices: Some Useful Lists

Chapter 4. The Formation of Plurals

1. There are three chief ways of forming the plural in English:

a.  By adding es or s to the singular. Examples: “box,” “boxes”; “gas,” “gases”; “loaf,” “loaves”; “witch,” “witches”; “shelf,” “shelves”; “hero,” “heroes”; “lady,” “ladies”; “thief,” “thieves.”
   Note the following:

  i. Nouns ending in f commonly change the f into v, but we say “roofs,” “cliffs,” “dwarfs,” “chiefs” and so on.

 ii. The letter y with a vowel before it is not changed in the plural. Examples: “keys,” “valleys,” “chimneys,” “days.”

iii. The letter y with a consonant before it is changed in the plural. Examples: “ladies,” “rubies,” “soliloquies.”

b.  By adding en. Examples: “oxen,” “children,” “brethren.”

c.  By changing the vowel sound. Examples: “man,” “men”; “foot,” “feet”; “goose,” “geese”; “tooth,” “teeth”; “mouse,” “mice”; “louse,” “lice.”

2. Some nouns are the same in the plural as in the singular. Examples: “deer,” “sheep,” “cod,” “trout,” “mackerel.”

3. A few words are false plurals: singulars which look like plurals. Examples: “alms” (meaning a donation) and “riches” (meaning “wealth”).

4. A few plurals are treated as singulars. Examples: “amends” (as in “make much—not many—amends”); “gallows” (as in “more than one gallows”); “news” (as in “much news”); “odds” (as in “it doesn’t make much odds”); “pains” (as in “take much pains”); “shambles” (as in the cliché “one almighty shambles”).

5. Many nouns can be used only in the plural. They fall into two categories:

a.  Names of things consisting of two or more parts. These are “bellows,” “drawers,” “jeans,” “lungs,” “pincers,” “pliers,” “scissors,” “shears,” “snuffers,” “spectacles” and “glasses” (meaning “spectacles”), “tongs,” “trousers” and “tweezers.” Some of these, such as “drawer,” “glass” and “spectacle,” change their meaning entirely in the singular.

b.  Names of things that are taken in the mass. These are “annals,” “archives,” “ashes,” “assets,” “dregs,” “embers,” “entrails,” “hustings,” “lees” (meaning the sediment of wine), “measles,” “molasses,” “mumps,” “oats,” “staggers” and “victuals.”

6. Many nouns have been brought into the English language from other languages, some of them naturalised and having adopted English plurals, some of them keeping their original plurals.

a.  Examples of imports that now have, or at least can have, English plurals: bandits (originally “banditti”), “cherubs” and “seraphs” (“cherubim” and “seraphim”), “dogmas” (“dogmata”), “focuses” (“foci”), “formulas” (“formulae”), “indexes” (“indices”) in one of the senses of the word “index” (see below), “memorandums” (“memoranda”) and “terminuses” (“terminae”).

b.  Examples of imports keep, or at least can keep, their own plurals, not always perfectly:

  i. Latin words: “datum,” “data”; “formula,” “formulae”; “genus,” “genera”; “series,” “series”; “species,” “species”; and “stratum,” “strata.”

 ii. Greek words: “analysis,” “analyses”; “axis,” “axes”; “ellipsis,” “ellipses”; “parenthesis,” “parentheses”; and “phenomenon,” “phenomena.”

iii. French words: “monsieur,” “messieurs”; and “madame,” “mesdames.”

iv. Italian words: “bandit,” “banditti”; “dilettante,” “dilettanti”; “libretto,” “libretti”; and “virtuoso,” “virtuosi.”

 v. Hebrew words: “cherub,” “cherubim”; and “seraph,” “seraphim.”

7. Compound words sometimes put the first word into the plural and not the second, sometimes the second word and not the first, and sometimes both. There are two rules to show which category a compound word belongs to, and one category where one simply has to learn its very few instances:

a.  When one of the words is clearly the leading word. Examples: “sons-in-law,” “hangers-on” and “lookers-on.”
   (In the United States, “attorney general” and “major general” are most often treated as two words and not hyphenated, and the plural of “court-martial” is usually “courts-martial.”)

b.  It is difficult to identify a rule under which occasionally both parts of the compound are in the plural, as with “menservants” and “lords-justices.”

8. Finally, several nouns have two plurals, each giving a radically different meaning. Examples: “brother”: “brothers” (by blood) and “brethren” (of a community); “cloth”: “cloths” (kinds of cloth) and “clothes” (garments); “die”: “dies” (stamps for coining) and “dice” (cubes for gaming); “fish”: “fishes” (looked at separately) and “fish” (taken collectively); “genius”: “geniuses” (men of talent) and “genii” (powerful spirits); “index”: “indexes” (to books) and “indices” (to quantities in algebra); “penny”: “pennies” (taken separately) and “pence” (taken collectively); “shot”: “shots” (separate discharges) and “shot” (pellets, collectively).