Plurals take many (sometimes curious or counterintuitive) forms. Here is an outline of how to form various types of plurals according to the word form or ending:
For words ending in:
- nonsibilant, or voiceless, consonants: add -s (dogs).
- voiced, or sibilant, consonant blends: add -es (riches).
- vowels: add -s (knees).
- -f: delete -f and add -ves (loaves) or -s (chiefs).
- -x: add -es (foxes).
- -y: delete -y and add -ies (bodies).
For words of Latin or Greek origin ending in:
- -a: add -s or -e, depending on context (formulas/formulae).
- -ex: add -es or delete -x and add -ces, depending on context (indexes/indices).
- -ies: leave as is (species).
- -is: change to -es (axis).
- -ma: add -s or -ata, depending on context (stigmas/stigmata).
- -um: add -s (aquariums), delete -um and add -a (curricula), or either depending on context (mediums/media).
- -us: delete -us and add -i (alumni), -ii (radii), -era (genera), or -ora (corpora), or leave -us and add -es ( octopuses) or use another form, depending on context (cactuses/cacti).
Types of irregular forms include:
- ablaut, or mutated, plurals, with changes in the midst of a word (tooth/teeth; louse/lice).
- identical singular and plural forms (deer, spacecraft).
- -en and -ren endings (oxen, children).
Some words have more than one plural form, one of which may be archaic (cows/kine) or reserved for a distinct meaning (dice/dies).
One problematic category is in referring to fish in general and specific varieties of fish in particular: “Look at all the fish!” but “Gars are long, slender, predatory fishes.” Also, some types are given a distinct plural form (sharks, barracudas), though for others, the plural form is identical to the singular one (salmon, sturgeon).
Plurals of letters, numbers, or abbreviations are generally formed simply by adding -s, although occasionally, the otherwise incorrect use of an interceding apostrophe is warranted (“Mind your p’s and q’s”). Usually, however, pluralizing names of letters requires no special treatment, especially when the letter is italicized, as here: “There are five es in beekeeper.” Initialisms and acronyms also require only an -s: “Many NGOs are headquartered here,” “It’s like having two NASAs.”
The singular forms of some words are, because they end in -s, sometimes mistaken as plurals, and the letter is then incorrectly deleted to create a false singular form. Examples include gyros, kudos, and biceps (as well as triceps and quadriceps). The plural forms are the same, although the latter examples are also pluralized by adding -es. For other words, the original singular has been supplanted by a plural form used in both cases (alga by algae; graffito by graffiti).